Paper (https://snwh.org/paper/theme) is a particularly great theme for the Gnome desktop (it also works with Budgie, Pantheon, and Unity, but I haven't tried it on any of those). The author's icon set is also pretty nice.

Wow! Definitely one of the best designs I've seen in the OSS world. Finally someone with actual design chops has risen the bar!

It lacks contrast. The fonts in particular are too thin and hard to read for my liking.

That really is quite nice. Looks great in Elementary.

Quick googling "material theme gtk" returns this which looks very nice actually https://github.com/nana-4/Flat-Plat

oh no. Another "flat design" theme. A button must be distinguishable from other design elements in my opinion. I hope this fad goes away soon, it's even more annoying than the glossy look or skeuomorphs.

Nothing is more annoying than skeumorphs. :)

It should be distinguishable, not just from the theme. A good UX/UI design will make that clear regardless.