css: Move default font handling

css: Move default font handling

The default font is no longer handled like a custom style sheet that
overrides everything, but as the initial value. This is the same
behavior as in web browsers.

And it allows the theme to actually use the 'font-family' and
'font-size' properties. Of course, a well behaved theme will respect the
setting as much as possible and for example use relative font sizes
(which aren't yet supported, but will be soon).