I wasn't aware of this until the #4197 problem but gnome applications seem to use "symbolic icons" now (https://blog.jimmac.eu/2021/how-to-symbolic/) which might also be interesting for Geany instead of the currently used "action icons".
I for instance did run into an issue with "action icons" when I needed an icon for tree expand/collapse. I used `GTK_STOCK_ADD` which normally looks like `+` which can be used as an expand symbol, but under some themes this icon shows some object over which the word `ADD` are shown. This clearly isn't something one wants to see for "expand".
On the other hand, "symbolic icons" usage is more based on how the actual icon looks like than what action it is supposed to represent. GTK itself provides some basic set of icons but there are quite many icons in the "icons library" application (https://flathub.org/apps/org.gnome.design.IconLibrary) - the extra icons have to be bundled with the application (which probably isn't a problem size-wise as these are small). The result is that the application has complete control over how the icons look like unlike semantic icons. The possible disadvantage is that these icons are black - or more precisely, single color, but I believe GTK allows these to be recolored to whatever other color.
Here are some screenshots of available icons - there are even some "development" icons which could be useful for things like "compile", "build", etc. (This is not all of the available icons, there are several pages of them)
<img width="926" alt="Screenshot 2025-01-08 at 19 12 50" src="https://github.com/user-attachments/assets/d3a9932c-1f8a-4f84-9782-55ed562555a3" />
<img width="926" alt="Screenshot 2025-01-08 at 19 20 50" src="https://github.com/user-attachments/assets/9a096a66-556f-405d-9e9e-09a6efd2918a" />