use bold font for headings, refs #4451

git-svn-id: svn://develop.studip.de/studip/trunk@30830 c5b85792-5220-0410-ad23-ed5b6b8ad999
redefine font-weights, refs #4451

git-svn-id: svn://develop.studip.de/studip/trunk@30831 c5b85792-5220-0410-ad23-ed5b6b8ad999