One of the posters on my forum reported that they were seeing scrollbars within the posts using the default themes (and various colour default themes). I have apparently managed to solve the problem with a rather simple change.
If you open the style.css file and look for .post, .personalmessage you should find the overflow-parameter set to auto. Change this to visible and save the file. It should resolve the problem. I would appreciate some feedback on this in case you can confirm that this works.
That works just fine, but you can just get rid of the "overflow:auto" line altogether if you don't want it.