I'm working on a GTK3 and 4 theme that adapts to @kde Plasma color scheme (riding on the trick that Breeze GTK uses), that looks very much like Oxygen (although with some modifications, such as colored toggled buttons). So far, it's still in alpha, therefore not published in opendesktop, but the repo is already public and clonable: https://git.disroot.org/eudaimon/oxygen-adaptive
Still a WIP: I want to improve notebooks and checks/radios.
Despite being alpha, as far as I've tested, GTK3 and 4 are very usable. GTK2 needs improvement (although it's also usable), and I'll try to implement what I did in my ClearCrystal theme (https://www.pling.com/p/2013507/), which is a template model so gtk2 can at least be easily themeable. I'll look into what Plasma does for GTK2, actually, maybe it's the solution!
The preview image is from ClearCrystal, another nice theme (and not #skeuomorphic, for a change!)