[nine_k]

I've spent considerable time looking for GTK3 themes that meet your requirements.

My current choice is openSuse-Pistachio which is green, reasonably compact, and did not break for two consecutive Ubuntu releases. zonColor icons work beautifully, too, with adjustable colors.

There are a few more reasonable choices (though not oodles) on gnome-look.org

[weland]

Thanks! I'll keep that in mind for when I run out of choices and have to upgrade :).