Global Settings & Styles (theme.json)

WordPress 5.8 comes with a new mechanism to configure the editor that enables a finer-grained control and introduces the first step in managing styles for future WordPress releases: the theme.json file. This page documents its format.

Rationale
- Settings for the block editor
- Settings can be controlled per block
- Styles are managed
- CSS Custom Properties: presets & custom
Specification
- version
- settings
  - Backward compatibility with add_theme_support
  - Presets
  - Custom
  - Setting examples
- styles
  - Top-level
  - Block-level
  - Elements
- customTemplates
- templateParts
FAQ
- The naming schema of CSS Custom Properties
- Why using -- as a separator?
- How settings under "custom" create new CSS Custom Properties

Rationale

The Block Editor API has evolved at different velocities and there are some growing pains, specially in areas that affect themes. Examples of this are: the ability to control the editor programmatically, or a block style system that facilitates user, theme, and core style preferences.

This describes the current efforts to consolidate the various APIs related to styles into a single point – a theme.json file that should be located inside the root of the theme directory.

Settings for the block editor

Instead of the proliferation of theme support flags or alternative methods, the theme.json files provides a canonical way to define the settings of the block editor. These settings includes things like:

What customization options should be made available or hidden from the user.
What are the default colors, font sizes... available to the user.
Defines the default layout of the editor (widths and available alignments).

Settings can be controlled per block

For more granularity, these settings also work at the block level in theme.json.

Examples of what can be achieved are:

Use a particular preset for a block (e.g.: table) but the common one for the rest of blocks.
Enable font size UI controls for all blocks but the headings block.
etc.

Styles are managed

By using the theme.json file to set style properties in a structured way, the Block Editor can "manage" the CSS that comes from different origins (user, theme, and core CSS). For example, if a theme and a user set the font size for paragraphs, we only enqueue the style coming from the user and not the theme's.

Some of the advantages are:

Reduce the amount of CSS enqueued.
Prevent specificity wars.

CSS Custom Properties: presets & custom

There are some areas of styling that would benefit from having shared values that can change across a site.

To address this need, we've started to experiment with CSS Custom Properties, aka CSS Variables, in some places:

Presets: color palettes, font sizes, or gradients declared by the theme are converted to CSS Custom Properties and enqueued both the front-end and the editors.

{% codetabs %} {% Input %}

{
	"version": 1,
	"settings": {
		"color": {
			"palette": [
				{
					"name": "Black",
					"slug": "black",
					"color": "#000000"
				},
				{
					"name": "White",
					"slug": "white",
					"color": "#ffffff"
				}
			]
		}
	}
}

{% Output %}

body {
	--wp--preset--color--black: #000000;
	--wp--preset--color--white: #ffffff;
}

{% end %}

Custom properties: there's also a mechanism to create your own CSS Custom Properties.

{% codetabs %} {% Input %}

{
	"version": 1,
	"settings": {
		"custom": {
			"line-height": {
				"body": 1.7,
				"heading": 1.3
			}
		}
	}
}

{% Output %}

body {
	--wp--custom--line-height--body: 1.7;
	--wp--custom--line-height--heading: 1.3;
}

{% end %}

Specification

This specification is the same for the three different origins that use this format: core, themes, and users. Themes can override core's defaults by creating a file called theme.json. Users, via the site editor, will also be also to override theme's or core's preferences via an user interface that is being worked on.

{
	"version": 1,
	"settings": {},
	"styles": {},
	"customTemplates": {},
	"templateParts": {}
}

Version

This field describes the format of the theme.json file. The current and only version is 1.

WordPress 5.8 will ignore the contents of any theme.json whose version is not equals to the current. Should the Gutenberg plugin need it, it'll update the version and will add the corresponding migration mechanisms from older versions.

Settings

The Gutenberg plugin extends the settings available from WordPress 5.8, so they can be used with other WordPress versions and they go through a maturation process before being ported to core.

The tabs below show WordPress 5.8 supported settings and the ones supported by the Gutenberg plugin.

The settings section has the following structure:

{% codetabs %} {% WordPress %}

{
	"version": 1,
	"settings": {
		"border": {
			"customRadius": false
		},
		"color": {
			"custom": true,
			"customDuotone": true,
			"customGradient": true,
			"duotone": [],
			"gradients": [],
			"link": false,
			"palette": []
		},
		"custom": {},
		"layout": {
			"contentSize": "800px",
			"wideSize": "1000px"
		},
		"spacing": {
			"customMargin": false,
			"customPadding": false,
			"units": [ "px", "em", "rem", "vh", "vw" ]
		},
		"typography": {
			"customFontSize": true,
			"customLineHeight": false,
			"dropCap": true,
			"fontSizes": []
		},
		"blocks": {
			"core/paragraph": {
				"color": {},
				"custom": {},
				"layout": {},
				"spacing": {},
				"typography": {}
			},
			"core/heading": {},
			"etc": {}
		}
	}
}

{% Gutenberg %}

{
	"version": 1,
	"settings": {
		"border": {
			"color": false,
			"radius": false,
			"style": false,
			"width": false
		},
		"color": {
			"background": true,
			"custom": true,
			"customDuotone": true,
			"customGradient": true,
			"defaultGradients": true,
			"defaultPalette": true,
			"duotone": [],
			"gradients": [],
			"link": false,
			"palette": [],
			"text": true
		},
		"custom": {},
		"layout": {
			"contentSize": "800px",
			"wideSize": "1000px"
		},
		"spacing": {
			"blockGap": null,
			"margin": false,
			"padding": false,
			"units": [ "px", "em", "rem", "vh", "vw" ]
		},
		"typography": {
			"customFontSize": true,
			"dropCap": true,
			"fontFamilies": [],
			"fontSizes": [],
			"fontStyle": true,
			"fontWeight": true,
			"letterSpacing": true,
			"lineHeight": false,
			"textDecoration": true,
			"textTransform": true
		},
		"blocks": {
			"core/paragraph": {
				"border": {},
				"color": {},
				"custom": {},
				"layout": {},
				"spacing": {},
				"typography": {}
			},
			"core/heading": {},
			"etc": {}
		}
	}
}

{% end %}

Each block can configure any of these settings separately, providing a more fine-grained control over what exists via add_theme_support. The settings declared at the top-level affect to all blocks, unless a particular block overwrites it. It's a way to provide inheritance and configure all blocks at once.

Note, however, that not all settings are relevant for all blocks. The settings section provides an opt-in/opt-out mechanism for themes, but it's the block's responsibility to add support for the features that are relevant to it. For example, if a block doesn't implement the dropCap feature, a theme can't enable it for such a block through theme.json.

Backward compatibility with add_theme_support

To retain backward compatibility, the existing add_theme_support declarations that configure the block editor are retrofit in the proper categories for the top-level section. For example, if a theme uses add_theme_support('disable-custom-colors'), it'll be the same as setting settings.color.custom to false. If the theme.json contains any settings, these will take precedence over the values declared via add_theme_support. This is the complete list of equivalences:

add_theme_support	theme.json setting
`custom-line-height`	Set `typography.lineHeight` to `true`.
`custom-spacing`	Set `spacing.padding` to `true`.
`custom-units`	Provide the list of units via `spacing.units`.
`disable-custom-colors`	Set `color.custom` to `false`.
`disable-custom-font-sizes`	Set `typography.customFontSize` to `false`.
`disable-custom-gradients`	Set `color.customGradient` to `false`.
`editor-color-palette`	Provide the list of colors via `color.palette`.
`editor-font-sizes`	Provide the list of font size via `typography.fontSizes`.
`editor-gradient-presets`	Provide the list of gradients via `color.gradients`.
`experimental-link-color`	Set `color.link` to `true`.

Presets

Presets are part of the settings section. They are values that are shown to the user via some UI controls. By defining them via theme.json the engine can do more for themes, such as automatically translate the preset name or enqueue the corresponding CSS classes and custom properties.

The following presets can be defined via theme.json:

color.duotone: doesn't generate classes or custom properties.
color.gradients: generates a single class and custom property per preset value.
color.palette:
- generates 3 classes per preset value: color, background-color, and border-color.
- generates a single custom property per preset value
typography.fontSizes: generates a single class and custom property per preset value.
typography.fontFamilies: generates a single custom property per preset value.

The naming schema for the classes and the custom properties is as follows:

Custom Properties: --wp--preset--{preset-category}--{preset-slug} such as --wp--preset--color--black
Classes: .has-{preset-slug}-{preset-category} such as .has-black-color.

{% codetabs %} {% Input %}

{
	"version": 1,
	"settings": {
		"color": {
			"duotone": [
				{
					"colors": [ "#000", "#FFF" ],
					"slug": "black-and-white",
					"name": "Black and White"
				}
			],
			"gradients": [
				{
					"slug": "blush-bordeaux",
					"gradient": "linear-gradient(135deg,rgb(254,205,165) 0%,rgb(254,45,45) 50%,rgb(107,0,62) 100%)",
					"name": "Blush bordeaux"
				},
				{
					"slug": "blush-light-purple",
					"gradient": "linear-gradient(135deg,rgb(255,206,236) 0%,rgb(152,150,240) 100%)",
					"name": "Blush light purple"
				}
			],
			"palette": [
				{
					"slug": "strong-magenta",
					"color": "#a156b4",
					"name": "Strong magenta"
				},
				{
					"slug": "very-dark-grey",
					"color": "rgb(131, 12, 8)",
					"name": "Very dark grey"
				}
			]
		},
		"typography": {
			"fontFamilies": [
				{
					"fontFamily": "-apple-system,BlinkMacSystemFont,\"Segoe UI\",Roboto,Oxygen-Sans,Ubuntu,Cantarell, \"Helvetica Neue\",sans-serif",
					"slug": "system-font",
					"name": "System Font"
				},
				{
					"fontFamily": "Helvetica Neue, Helvetica, Arial, sans-serif",
					"slug": "helvetica-arial",
					"name": "Helvetica or Arial"
				}
			],
			"fontSizes": [
				{
					"slug": "normal",
					"size": 16,
					"name": "Normal"
				},
				{
					"slug": "big",
					"size": 32,
					"name": "Big"
				}
			]
		},
		"blocks": {
			"core/group": {
				"color": {
					"palette": [
						{
							"slug": "black",
							"color": "#000000",
							"name": "Black"
						},
						{
							"slug": "white",
							"color": "#ffffff",
							"name": "White"
						},
					]
				}
			}
		}
	}
}

{% Output %}

/* Top-level custom properties */
body {
	--wp--preset--color--strong-magenta: #a156b4;
	--wp--preset--color--very-dark-grey: #444;
	--wp--preset--gradient--blush-bordeaux: linear-gradient( 135deg, rgb( 254, 205, 165 ) 0%, rgb( 254, 45, 45 ) 50%, rgb( 107, 0, 62 ) 100% );
	--wp--preset--gradient--blush-light-purple: linear-gradient( 135deg, rgb( 255, 206, 236 ) 0%, rgb( 152, 150, 240 ) 100% );
	--wp--preset--font-size--big: 32;
	--wp--preset--font-size--normal: 16;
	--wp--preset--font-family--helvetica-arial: Helvetica Neue, Helvetica, Arial, sans-serif;
	--wp--preset--font-family--system: -apple-system,BlinkMacSystemFont,\"Segoe UI\",Roboto,Oxygen-Sans,Ubuntu,Cantarell, \"Helvetica Neue\",sans-serif;
}

/* Block-level custom properties (bounded to the group block) */
.wp-block-group {
	--wp--preset--color--black: #000000;
	--wp--preset--color--white: #ffffff;
}

/* Top-level classes */
.has-strong-magenta-color { color: #a156b4 !important; }
.has-strong-magenta-background-color { background-color: #a156b4 !important; }
.has-strong-magenta-border-color { border-color: #a156b4 !important; }
.has-very-dark-grey-color { color: #444 !important; }
.has-very-dark-grey-background-color { background-color: #444 !important; }
.has-very-dark-grey-border-color { border-color: #444 !important; }
.has-blush-bordeaux-background { background: linear-gradient( 135deg, rgb( 254, 205, 165 ) 0%, rgb( 254, 45, 45 ) 50%, rgb( 107, 0, 62 ) 100% ) !important; }
.has-blush-light-purple-background { background: linear-gradient( 135deg, rgb( 255, 206, 236 ) 0%, rgb( 152, 150, 240 ) 100% ) !important; }
.has-big-font-size { font-size: 32; }
.has-normal-font-size { font-size: 16; }

/* Block-level classes (bounded to the group block) */
.wp-block-group.has-black-color { color: #a156b4 !important; }
.wp-block-group.has-black-background-color { background-color: #a156b4 !important; }
.wp-block-group.has-black-border-color { border-color: #a156b4 !important; }
.wp-block-group.has-white-color { color: #444 !important; }
.wp-block-group.has-white-background-color { background-color: #444 !important; }
.wp-block-group.has-white-border-color { border-color: #444 !important; }

{% end %}

To maintain backward compatibility, the presets declared via add_theme_support will also generate the CSS Custom Properties. If the theme.json contains any presets, these will take precedence over the ones declared via add_theme_support.

Preset classes are attached to the content of a post by some user action. That's why the engine will add !important to these, because user styles should take precedence over theme styles.

Custom

In addition to create CSS Custom Properties for the presets, the theme.json also allows for themes to create their own, so they don't have to be enqueued separately. Any values declared within the custom field will be transformed to CSS Custom Properties following this naming schema: --wp--custom--<variable-name>.

For example:

{% codetabs %} {% Input %}

{
	"version": 1,
	"settings": {
		"custom": {
			"baseFont": 16,
			"lineHeight": {
				"small": 1.2,
				"medium": 1.4,
				"large": 1.8
			}
		},
		"blocks": {
			"core/group": {
				"custom": {
					"baseFont": 32
				}
			}
		}
	}
}

{% Output %}

body {
	--wp--custom--base-font: 16;
	--wp--custom--line-height--small: 1.2;
	--wp--custom--line-height--medium: 1.4;
	--wp--custom--line-height--large: 1.8;
}
.wp-block-group {
	--wp--custom--base-font: 32;
}

{% end %}

Note that the name of the variable is created by adding -- in between each nesting level and camelCase fields are transformed to kebab-case.

Settings examples

Enable custom colors only for the paragraph block:

{
	"version": 1,
	"settings": {
		"color": {
			"custom": false
		},
		"blocks": {
			"core/paragraph": {
				"color": {
					"custom": true
				}
			}
		}
	}
}

Disable border radius for the button block:

{
	"version": 1,
	"settings": {
		"blocks": {
			"core/button": {
				"border": {
					"customRadius": false
				}
			}
		}
	}
}

Provide the group block a different palette than the rest:

{
	"version": 1,
	"settings": {
		"color": {
			"palette": [
				{
					"slug": "black",
					"color": "#000000",
					"name": "Black"
				},
				{
					"slug": "white",
					"color": "#FFFFFF",
					"name": "White"
				},
				{
					"slug": "red",
					"color": "#FF0000",
					"name": "Red"
				},
				{
					"slug": "green",
					"color": "#00FF00",
					"name": "Green"
				},
				{
					"slug": "blue",
					"color": "#0000FF",
					"name": "Blue"
				}
			]
		},
		"blocks": {
			"core/group": {
				"color": {
					"palette": [
						{
							"slug": "black",
							"color": "#000000",
							"name": "Black"
						},
						{
							"slug": "white",
							"color": "#FFF",
							"name": "White"
						}
					]
				}
			}
		}
	}
}

Styles

The Gutenberg plugin extends the styles available from WordPress 5.8, so they can be used with other WordPress versions and they go through a maturation process before being ported to core.

The tabs below show WordPress 5.8 supported styles and the ones supported by the Gutenberg plugin.

Each block declares which style properties it exposes via the block supports mechanism. The support declarations are used to automatically generate the UI controls for the block in the editor. Themes can use any style property via the theme.json for any block ― it's the theme's responsibility to verify that it works properly according to the block markup, etc.

{% codetabs %}

{% WordPress %}

{
	"version": 1,
	"styles": {
		"border": {
			"radius": "value"
		},
		"color": {
			"background": "value",
			"gradient": "value",
			"text": "value"
		},
		"spacing": {
			"margin": {
				"top": "value",
				"right": "value",
				"bottom": "value",
				"left": "value"
			},
			"padding": {
				"top": "value",
				"right": "value",
				"bottom": "value",
				"left": "value"
			}
		},
		"typography": {
			"fontSize": "value",
			"lineHeight": "value"
		},
		"elements": {
			"link": {
				"border": {},
				"color": {},
				"spacing": {},
				"typography": {}
			},
			"h1": {},
			"h2": {},
			"h3": {},
			"h4": {},
			"h5": {},
			"h6": {}
		},
		"blocks": {
			"core/group": {
				"border": {},
				"color": {},
				"spacing": {},
				"typography": {},
				"elements": {
					"link": {},
					"h1": {},
					"h2": {},
					"h3": {},
					"h4": {},
					"h5": {},
					"h6": {}
				}
			},
			"etc": {}
		}
	}
}

{% Gutenberg %}

{
	"version": 1,
	"styles": {
		"border": {
			"color": "value",
			"radius": "value",
			"style": "value",
			"width": "value"
		},
		"color": {
			"background": "value",
			"gradient": "value",
			"text": "value"
		},
		"filter": {
			"duotone": "value"
		},
		"spacing": {
			"blockGap": "value",
			"margin": {
				"top": "value",
				"right": "value",
				"bottom": "value",
				"left": "value"
			},
			"padding": {
				"top": "value",
				"right": "value",
				"bottom": "value",
				"left": "value"
			}
		},
		"typography": {
			"fontFamily": "value",
			"fontSize": "value",
			"fontStyle": "value",
			"fontWeight": "value",
			"letterSpacing": "value",
			"lineHeight": "value",
			"textDecoration": "value",
			"textTransform": "value"
		},
		"elements": {
			"link": {
				"border": {},
				"color": {},
				"spacing": {},
				"typography": {}
			},
			"h1": {},
			"h2": {},
			"h3": {},
			"h4": {},
			"h5": {},
			"h6": {}
		},
		"blocks": {
			"core/group": {
				"border": {},
				"color": {},
				"spacing": {},
				"typography": {},
				"elements": {
					"link": {},
					"h1": {},
					"h2": {},
					"h3": {},
					"h4": {},
					"h5": {},
					"h6": {}
				}
			},
			"etc": {}
		}
	}
}

{% end %}

Top-level styles

Styles found at the top-level will be enqueued using the body selector.

{% codetabs %} {% Input %}

{
	"version": 1,
	"styles": {
		"color": {
			"text": "var(--wp--preset--color--primary)"
		}
	}
}

{% Output %}

body {
	color: var( --wp--preset--color--primary );
}

{% end %}

Block styles

Styles found within a block will be enqueued using the block selector.

By default, the block selector is generated based on its name such as .wp-block-<blockname-without-namespace>. For example, .wp-block-group for the core/group block. There are some blocks that want to opt-out from this default behavior. They can do so by explicitly telling the system which selector to use for them via the __experimentalSelector key within the supports section of its block.json file.

{% codetabs %} {% Input %}

{
	"version": 1,
	"styles": {
		"color": {
			"text": "var(--wp--preset--color--primary)"
		},
		"blocks": {
			"core/paragraph": {
				"color": {
					"text": "var(--wp--preset--color--secondary)"
				}
			},
			"core/group": {
				"color": {
					"text": "var(--wp--preset--color--tertiary)"
				}
			}
		}
	}
}

{% Output %}

body {
	color: var( --wp--preset--color--primary );
}
p { /* The core/paragraph opts out from the default behaviour and uses p as a selector. */
	color: var( --wp--preset--color--secondary );
}
.wp-block-group {
	color: var( --wp--preset--color--tertiary );
}

{% end %}

Element styles

In addition to top-level and block-level styles, there's the concept of elements that can used in both places. There's a closed set of them:

link: maps to the a CSS selector.
h1: maps to the h1 CSS selector.
h2: maps to the h2 CSS selector.
h3: maps to the h3 CSS selector.
h4: maps to the h4 CSS selector.
h5: maps to the h5 CSS selector.
h6: maps to the h6 CSS selector.

If they're found in the top-level the element selector will be used. If they're found within a block, the selector to be used will be the element's appended to the corresponding block.

{% codetabs %} {% Input %}

{
	"version": 1,
	"styles": {
		"typography": {
			"fontSize": "var(--wp--preset--font-size--normal)"
		},
		"elements": {
			"h1": {
				"typography": {
					"fontSize": "var(--wp--preset--font-size--huge)"
				}
			},
			"h2": {
				"typography": {
					"fontSize": "var(--wp--preset--font-size--big)"
				}
			},
			"h3": {
				"typography": {
					"fontSize": "var(--wp--preset--font-size--medium)"
				}
			}
		},
		"blocks": {
			"core/group": {
				"elements": {
					"h2": {
						"typography": {
							"fontSize": "var(--wp--preset--font-size--small)"
						}
					},
					"h3": {
						"typography": {
							"fontSize": "var(--wp--preset--font-size--smaller)"
						}
					}
				}
			}
		}
	}
}

{% Output %}

body {
	font-size: var( --wp--preset--font-size--normal );
}
h1 {
	font-size: var( --wp--preset--font-size--huge );
}
h2 {
	font-size: var( --wp--preset--font-size--big );
}
h3 {
	font-size: var( --wp--preset--font-size--medium );
}
.wp-block-group h2 {
	font-size: var( --wp--preset--font-size--small );
}
.wp-block-group h3 {
	font-size: var( --wp--preset--font-size--smaller );
}

{% end %}

customTemplates

This field is only allowed when the Gutenberg plugin is active. In WordPress 5.8 will be ignored.

Within this field themes can list the custom templates present in the block-templates folder. For example, for a custom template named my-custom-template.html, the theme.json can declare what post types can use it and what's the title to show the user:

name: mandatory.
title: mandatory, translatable.
postTypes: optional, only applies to the page by default.

{
    "version": 1,
	"customTemplates": [
		{
			"name": "my-custom-template",
			"title": "The template title",
			"postTypes": [
				"page",
				"post",
				"my-cpt"
			]
		}
	]
}

templateParts

This field is only allowed when the Gutenberg plugin is active. In WordPress 5.8 will be ignored.

Within this field themes can list the template parts present in the block-template-parts folder. For example, for a template part named my-template-part.html, the theme.json can declare the area term for the template part entity which is responsible for rendering the corresponding block variation (Header block, Footer block, etc.) in the editor. Defining this area term in the json will allow the setting to persist across all uses of that template part entity, as opposed to a block attribute that would only affect one block. Defining area as a block attribute is not recommended as this is only used 'behind the scenes' to aid in bridging the gap between placeholder flows and entity creation.

Currently block variations exist for "header" and "footer" values of the area term, any other values and template parts not defined in the json will default to the general template part block. Variations will be denoted by specific icons within the editor's interface, will default to the corresponding semantic HTML element for the wrapper (this can also be overridden by the tagName attribute set on the template part block), and will contextualize the template part allowing more custom flows in future editor improvements.

name: mandatory.
title: optional, translatable.
area: optional, will be set to uncategorized by default and trigger no block variation.

{
    "version": 1,
	"templateParts": [
		{
			"name": "my-template-part",
			"title": "Header",
			"area": "header"
		}
	]
}

Developing with theme.json

It can be difficult to remember the theme.json settings and properties while you develop, so a JSON scheme was created to help. The schema is available at https://schemas.wp.org/trunk/theme.json

Code editors can pick up the schema and can provide help like tooltips, autocomplete, or schema validation in the editor. To use the schema in Visual Studio Code, add "$schema": "https://schemas.wp.org/trunk/theme.json" to the beginning of your theme.json file.

Frequently Asked Questions

The naming schema of CSS Custom Properties

One thing you may have noticed is the naming schema used for the CSS Custom Properties the system creates, including the use of double hyphen, --, to separate the different "concepts". Take the following examples.

Presets such as --wp--preset--color--black can be divided into the following chunks:

--wp: prefix to namespace the CSS variable.
preset : indicates is a CSS variable that belongs to the presets.
color: indicates which preset category the variable belongs to. It can be color, font-size, gradients.
black: the slug of the particular preset value.

Custom properties such as --wp--custom--line-height--body, which can be divided into the following chunks:

--wp: prefix to namespace the CSS variable.
custom: indicates is a "free-form" CSS variable created by the theme.
line-height--body: the result of converting the "custom" object keys into a string.

The -- as a separator has two functions:

Readability, for human understanding. It can be thought as similar to the BEM naming schema, it separates "categories".
Parsability, for machine understanding. Using a defined structure allows machines to understand the meaning of the property --wp--preset--color--black: it's a value bounded to the color preset whose slug is "black", which then gives us room to do more things with them.

Why using `--` as a separator?

We could have used any other separator, such as a single -.

However, that'd have been problematic, as it'd have been impossible to tell how --wp-custom-line-height-template-header should be converted back into an object, unless we force theme authors not to use - in their variable names.

By reserving -- as a category separator and let theme authors use - for word-boundaries, the naming is clearer: --wp--custom--line-height--template-header.

How settings under "custom" create new CSS Custom Properties

The algorithm to create CSS Variables out of the settings under the "custom" key works this way:

This is for clarity, but also because we want a mechanism to parse back a variable name such --wp--custom--line-height--body to its object form in theme.json. We use the same separation for presets.

For example:

{% codetabs %} {% Input %}

{
	"version": 1,
	"settings": {
		"custom": {
			"lineHeight": {
				"body": 1.7
			},
			"font-primary": "-apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen-Sans, Ubuntu, Cantarell, 'Helvetica Neue', sans-serif"
		}
	}
}

{% Output %}

body {
	--wp--custom--line-height--body: 1.7;
	--wp--custom--font-primary: "-apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen-Sans, Ubuntu, Cantarell, 'Helvetica Neue', sans-serif";
}

{% end %}

A few notes about this process:

camelCased keys are transformed into its kebab-case form, as to follow the CSS property naming schema. Example: lineHeight is transformed into line-height.
Keys at different depth levels are separated by --. That's why line-height and body are separated by --.
You shouldn't use -- in the names of the keys within the custom object. Example, don't do this:

{
	"version": 1,
	"settings": {
		"custom": {
			"line--height": { // DO NOT DO THIS
				"body": 1.7
			}
		}
	}
}

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theme-json.md

theme-json.md

Global Settings & Styles (theme.json)

Rationale

Settings for the block editor

Settings can be controlled per block

Styles are managed

CSS Custom Properties: presets & custom

Specification

Version

Settings

Backward compatibility with add_theme_support

Presets

Custom

Settings examples

Styles

Top-level styles

Block styles

Element styles

customTemplates

templateParts

Developing with theme.json

Frequently Asked Questions

The naming schema of CSS Custom Properties

Why using `--` as a separator?

How settings under "custom" create new CSS Custom Properties

Files

theme-json.md

Latest commit

History

theme-json.md

File metadata and controls

Global Settings & Styles (theme.json)

Rationale

Settings for the block editor

Settings can be controlled per block

Styles are managed

CSS Custom Properties: presets & custom

Specification

Version

Settings

Backward compatibility with add_theme_support

Presets

Custom

Settings examples

Styles

Top-level styles

Block styles

Element styles

customTemplates

templateParts

Developing with theme.json

Frequently Asked Questions

The naming schema of CSS Custom Properties

Why using -- as a separator?

How settings under "custom" create new CSS Custom Properties

Why using `--` as a separator?