How to add arbitrary custom CSS to a WordPress Block Theme using Site Editor?

In WordPress 6.1.1 (released 2022-11) this is not possible in the UI. But good news:

In WordPress 6.2 (to be released somewhen March 2023) this will be possible! Was introduced as an experimental feature in Gutenberg 14.8 from 2022-12 (see video )

It works like this:

  1. WordPress Dashboard > Appearance > Editor
  2. Open the Styles panel
  3. In the panel you got a new section “Additional CSS”.
  4. There you can paste arbitrary CSS. (and its immediately rendered on the canvas nearby)

How to add arbitrary custom CSS to a WordPress Block Theme using Site Editor