@Dim-Tim-1963 the FAQ is about installing themes on Windows, I thought you had installed a theme then tweaked that.
Editing `gtk.css` directly is not the recommended approach, for at least the reasons you noted, but also because it could be overwritten by upgrades. If you really wanted to "tweak" stuff then adding to `geany.css` is better since it shouldn't be overwritten as its in your user config.