Hi there,
my experience with CSS is that you have to take it slow, with small changes, and increment your customizations as you understand more of it.
What I found so far:
- the ~/.config/geany/geany.css file does not seem to have an influence on the tab styling
- the ~/.config/gtk-3.0/gtk.css file DOES override the gtk theme styling
I've attached the gtk.css file I used for my test to this message. When applied, together with my default GTK theme, WhiteSur-Dark-solid, it looks like this (on macos):
I hope this helps.