[FIX] bootstrap: better tablesorter.less mixin function to determine background to handle urls; use @input-bg for input background. (It is soooooo nice to be able to change once and have it automatically propagate to all less-generated css files!)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@51364 b456876b-0849-0410-b77d-98878d47e9d5