Updated doxygen configuration to reference ISIS .css files per Doxygen manual...

Updated doxygen configuration to reference ISIS .css files per Doxygen manual instead of using custom header files.  .css files were not being referenced corrently. Fixed some css problems so appearance is similar to old format. Simplified css configuration in favor of Doxygen config files.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@7506 41f8697f-d340-4b68-9986-7bafba869bb8