Theme is ClipStudio MidGrey from Zoldyako's Themes for Krita v2. I like it because it’s not too dark, not too light, background is completely neutral at 50, and thus doesn’t mess with value/color perception. Of course, it also matches what I use in CSP 
In addition to the right-click popup, I use every slot in Ten Brushes and swap them in and out with some regularity 
(Picture is a master study/brush test from Koteri Ink’s work).