doc/api: improve readability of the API docs

Still not great and probably makes any professional designer's eyes bleed, but
at least it's more readable now.

Changes:
- spacing after param name so they don't cuddle up with the description
- color changes and background image removals to drop the doxygen default look
- font size changes to not make things overrun
- font family change to make the function prototypes readable

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>