The mose.css file you are using appears to be a little old. Try using this one — current branch 1.9 mose.css — to see if the problem is fixed. Any modifications done to the mose.css you're currently using will have to be done to the new file, of course. WinMerge is a good (Windows) free program for comparing files and merging differences.
Actually security vulnerabilities have been found in Tiki sites older than 1.9.9, so updating overall would be a good idea if you haven't already done so.