More customizeable CSS rules

I’m trying to make a new theme, but the lack of customization (CSS rules) makes this almost impossible.
I’ve modified borders to be 15px and the shadow to be lighter, but now I’m stuck.

I wanted to put a slight linear gradient on the header bar, but there are no modifiable CSS rules for that, according to this. Most of the rules are hardcoded and adding them to a CSS and loading it won’t work, thanks to Polymer (sigh).
Changing the header font weight of a card is also not possible, because it’s hardcoded.
Changing the popup card radius is also not possible, because it’s hardcoded.
All of these things above I could do in the inspector window by modifying the CSS by hand, but this isn’t useful, 'cause on page load everything gets lost anyways.

A card modified by pure CSS

Well, my suggestion is following: Could we get normal CSS support, or at least all important CSS variables on all elements? Pretty please?

Thanks in advance