It appears that your CSS is one of the very, very limited few to be functional and maintained.
I was wondering if you could add in support for extending the back-primary/secondary into the main application.

So far I have yet to stumble across anything that works as well as your skin.

Very nice css, I like it a lot. I have noticed the underline color of the selected item in the navigation bar stays orange. It also happens with built-in dark theme but I think you might want to fix it in your css.

Here is the line to add into gpmcss.css :

.paper-tabs-1 #selectionBar.paper-tabs { border-color: white; }

Hey, thanks for writing this! Anyways, there is a small syntax error here. It's the semicolon before !important