themes: default: Use more compact representation of colors.

Regardless of if we use minification of the CSS files, these are beneficial
in any case.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
themes: default: Adjust some whitespace.

While more readable, we also may actually save some unnecessary bytes.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
themes: default: Remove px from 0px.

This makes the CSS'es work the same, while saving us some bytes (which can
be used for better purposes).

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>