On the Corkboard, only the transparency of the Status Stamps can be changed, not the color of any background colors. It’s actually possible to overrule the Corkboard pattern background in a Theme if you’re not cateful.
The best resource for changing any colors is currently the book “Mastering Scrivener”, but I’m writing “Mastering Scrivener Themes” at the moment, which will contain much more information about Themes than the three pages in the first book.
You can change all “Palette” colors to RGBA color notation in the QSS. You can also use Named Colors if you want.