When it comes to GTK themes, there are usually two problems.
- The theme doesn't keep up with GTK changes
- The theme is ugly
While the latter one might be subjective, you can't really argue against theme incompatibility. There are only a handful of themes which keep up with the GTK development cycle, greatly narrowing down your options.
Personally, I prefer my themes to be light/greyish and without any bright colors. From the few actively maintained themes (mentioned below), none really satisfies my demands. Fortunately the Zuki-Themes come pretty close to it.
I tweaked Zukitre a bit, replacing the highlight color and changing the check/radio buttons. The result is a pretty neutral and "boring" GTK theme which gets out of my way. Here's how it looks like.
Apart from my slightly tweaked Zukitre theme, here's a list of other maintained and complete (covering both GTK2 and 3) GTK themes:
-
Zuki-Themes, Plano Theme, Stilo Themes and Amber Theme
GTK2/3; 3.24+; actively maintained
These are just amazing and have the fewest issues out of the box (Zukitre from above is amongst these). -
Adwaita
GTK2/3; 3.20+; actively maintained
The default GTK theme which means it will always work. Could be a little bit less white/bright. -
Arc GTK2/3; 3.24+; actively maintained
Looks too bright due to the high contrast between the dark sidebars and pure white content areas. Grey text on white background is also slightly obnoxious. I also dislike the mixture of round and straight corners. -
Greybird
GTK2/3; 3.24+; actively maintained
Pretty easy on the eyes with its consistently grey color. The only downside is that it hasn't been ported (yet) to GTK 3.20.
If you know more themes which are complete and actively maintained, let me know and I'll add them to the list.