Just so you know: No devtools custom UI theme extension works. The flag to enable experimental stuff on devtools is there, but the option to use custom themes at devtools settings does nothing.
I also discovered that custom CSS rules are being applied to some dregree. The issue seems to arise from the devtools default theme overwriting any changes. If you use a CSS rule to set all elements backgrounds red you'll see it flash red for a second before being overlapped by the main container and all the styled elements.
I thought about going directly to the source, but there seems to be no way to easily access the default theme files. They must be packaged, not lying around as html and css files. I also had no luck tweaking devtools.js or devtools.html, the default theme is always applied last, overwriting anything.