FIX: make the icons available to implicit component

[12rambau]

Fix #982

apparently the :root pseudo selector is not available to :before. We never realized this because all our colors are stored in html since the dark theme introduction.

• make the icons available to all selectors: https://stackoverflow.com/a/38751906/6734243
• change the font to make the icon work

[12rambau]

anoher solution would be to replace the :root selector by htmleverywhere to avoid surprises (as we are using :before and :after a lot)

Looking on the web the :root is only useful if the css is used on multiple document including non-html (SVG, XML) which is not our case. In the context of pydata-sphinx-theme, :root is always html

[choldgraf]

I'm not really sure the implications between :root and :html but from your description, :html seems simpler and safe

[12rambau]

to make it simple, :root is useful if you use css for many file type as it will be replace by the first tag (html, div whatever). As we are building a theme for an html output (only) I think it's an overkill to use it and it leads to bugs of this type that are difficult to track down.

I replaced every occurences of :root by html everywhere (including documentation)

Note
custom.css that is using :root should still work as :root is html for the browser

[choldgraf]

I tested out the "hide search highlights" button and it works well! Looks good to me.