Hardcode theme to ipfire

This disables the theme support and makes it impossible to use any other
themes than the ipfire default theme.

The only intention of this patch is to hardcode the theme to ipfire.
To change any cgi we have is an ugly way, but the only way to do this
fast. The colour handling needs certainly to be improved as well, but
this will and should be done in other patches.

Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>