add customcss setting

The search and replace stuff could be simplified I think.
When you set up a setting you provide Moodle with a default setting,
yet most of the themes I see seem to check if its set and then
manually add this default value. Perhaps you can ask Moodle for it,
since it already knows what it is.
Merge pull request #189 from ds125v/customcss

add customcss setting