@Pepino, Whether this is a documentation issue or not depends on the theme being used. Some themes support custom.css and another theme might support custom.scss.
Themes do share commonalities but apart from a few minimal requirements they are totally free in their implementation.
Therefor, your suggested edit might work for your theme, but might fail in another…
Recipes should be taken as a guideline not as a final solution, which means tweaking and poking and some basic knowledge might be needed…
Thanks for the fast response. I use the standard theme Quark, so I thought it might be an issue. But with your explanation I’ll read the documentation a bit more open minded