[flowless]

Try Clearlooks-Phénix https://github.com/jpfleury/clearlooks-phenix

It's quite mature and available in number of distros. I'm pretty happy with it since I've discovered it.

[xpinguin]

I've encountered that while being on a quest to find perfect gtk3 theme :) Scrollbars are tremendous, which is crucial on tiling wm (see my screenshot above, for example).

[nycticorax]

I second this. Clearlooks-Phenix is great.