Colorize tabs - change colors

[pyinto]

Trying to change my tab's color in Sidebery since they aren't populated from Firefox Color extension's theme. I found this option:
Inherit custom (manually set) color in the child tabs
but don't really see where those colors are set? Would be great if somebody could point out exactly how and where those colors are located, so I can change styles manually.
I understand that color is set using tab-color property, but can't get exactly from where those default colors are extracted.

Thanks!