Thanks for explaining, @vkareh. Sounds like a tough situation.
I read that Gnome blog article. I have to admit that I’ve always had my color/theme customizations cause occasional bad indecipherable GUI widgets of too-similar/too-ugly colors on each other, even with GTK-2 (but I never reported bugs to particular apps about it, knowing it was due to my tweaks). GUI design isn’t my specialty so I can sympathize if comprehensive theming really does interfere with apps too much, but I tend to suspect that such a composability issue could be solved well somehow, but that would have to be some whole new different toolkit I guess.
My interest in easy changing of colors is mostly just to be able to change windows’ title bar and border colors semi-frequently, since those are what I like to use to indicate which window has the current focus (I’ve used Dopple for years), and colors and styles I don’t like make it pretty unpleasant. IIUC, this aspect is mostly separate from the theming of the windows’ content so it’d be nice if we could at least be able to customize this via a convenient GUI. Being mostly separate (IIUC), I hope the ability to change titlebar and border colors and styles doesn’t go away if they do away with theming.