You don't need a full CSS stylesheet just to change some colors. And no, you can't take CSS from a GTK app and use it with a web app or vice versa, the layout system and the elements/classes aren't compatible.
The trouble is that I don't want to hardcode the colors in the app, but to learn which theme the user is using and adapt the application to use the colors of that theme (and other features if possible)
If the toolkit supported this automatically, it would be easier to achieve this
That isn't possible in any scalable way in any toolkit. There's a good reason all the major commercial operating systems basically only support light/dark mode and maybe a couple other theme variants/options and that's it. I think you are underestimating just how complex the style is for a non-trivial framework/toolkit/application and how it's full of interdependencies and platform quirks.
I have no idea what you're referring to. Nothing changed, because it didn't actually work in the GTK2 days either. Actually it was worse then. Themes would break apps constantly. I remember always having to edit gtkrc when something broke, and sometimes that wouldn't even work or the app would just crash entirely. It was even more "fun" if a theme crashed the window manager or the panel and then you had to switch out to a VT to fix it.
You say you have no idea what I'm referring to, and then proceed to talk exactly about the feature I was refering to.
This together with your other inconsistent comments here make me doubt your sincerity. You seem to be either tracking Gtk/Gnome Dev closely or are a Dev, but then pretend to speak for users of Gnome and ask others not to speak for users of Gnome. You seem to be in some sort of damage control.
Gnome 2 came with themes, that worked really well, and rarely broke. Denying it only makes you look silly.
The themes didn't work well for me and I've talked to lots of other users who had the same experience. It was a broken feature and it was correct to remove it. If you're denying my experiences and telling me they're not real then you are the one who is pretending to speak for other users of GNOME. So please cut that out.
I don't follow development closer than anyone else, I just periodically read the dev blogs and changelogs like any user should. The difference is I don't assume that developers are hostile entities that are apparently spending all their spare time being passionate about their open source project just to annoy some users? Come now, think about it, isn't that a ridiculous notion? I mean really, I'm complaining about a broken feature and you're saying that's damage control. Wouldn't it be more "damage control" to insist that GNOME 2 somehow fixed all its theming bugs by doing something mysterious and unexplainable that no one can figure out 20 years later?
When I comment I try to counter the negativity and focus on making constructive comments, I strongly urge you to do the same. It's not like the developers are aliens that can't be understood by mortal humans, if you really need something explained you can just go in the Matrix channel and (respectfully) ask them questions.
If the toolkit supported this automatically, it would be easier to achieve this