fix css bug in less also, re #4272

git-svn-id: svn://develop.studip.de/studip/trunk@29861 c5b85792-5220-0410-ad23-ed5b6b8ad999
fix double slash bug in studip mixin, re #4272

git-svn-id: svn://develop.studip.de/studip/trunk@29862 c5b85792-5220-0410-ad23-ed5b6b8ad999