This is a short introduction on how to use CSS to style HTML documents converted from Markdown in DEVONthink. It contains some examples and is not intended to be a comprehensive introduction to CSS.
Instead, it should get you going with your own CSS if you are using DEVONthink and want something else than its default styles. The text originally appeared in several installations in the DEVONthink forum. It will not be updated there, so the current version will always be available here.
The complete text with all images and the accompanying CSS file are available as a zipped archive. After you downloaded and unpacked it, you can import the resulting folder into DEVONthink where it will be available as a self-contained group.
The accompanying CSS file styles.css is used to style the preview of the document in DEVONthink and of the PDF version that you can produce by simply converting Using CSS.md inside of DEVONthink. Alternatively, you can download the PDF directly. This CSS file might provide additional ideas on how write CSS rules, especially for print media.
Note that the text and the examples are geared towards the Markdown dialect used by DEVONthink, which is MultiMarkdown. It may or may not work with other Markdown implementations.