Yeah that seems like an area for improvement in Godot's GUI system indeed. For my stuff, I never had a need for theming, but good to keep in mind if one ever wanted to do an app shared with others.

Another homegrown personal tool I had once written was in Go with DearImgui, so basically with AllenDang/giu... also really quite rapid dev and lean-enough binary / perf.

But Godot is wysiwyg for GUI stuff, so that's a plus..