Update doxygen.css for Doxygen 1.8.6

doxygen.css was taken from `doxygen -g - | doxygen -` with one change:
All styles are restricted to #content.

For best results with Redmine Embedded, Doxygen must be configured with
SEARCHENGINE = NO and all other dynamic content left disabled.
Merge pull request #13 from alexhenrie/master

Update doxygen.css for Doxygen 1.8.6