Divide CSS properties between base styles and themes

Base styles should not include size, padding or positioning properties
that are likely to be overriden by the theme.

Bonus:
* Stop using UL/LI for select/option widgets - in no case were we ever
  using or want the browser's list styling, and we don't care about
  semantics at all

Change-Id: I056fc9c8f3ca5e9bde003ededb8e96e024311e20