I was just making a new subsite, and the very first thing I usually do is set the theme to match that of the parent. In my case this is a custom theme named CustomTheme. I selected it and applied it and BAM. The whole layout and style of the subsite was messed up. And the strange thing is that this now seems to happen with ALL the themes, all but the default one. I did not change anything to the main site or to the site collection, I just created a new subsite. And even now when I make another subsite elsewere in the site collection, this new subsite also suffers from the issue.
So what it comes to is that I can't apply any other theme than the default one. Does anyone perhaps know what may have caused this? I'm running SharePoint Foundation 2010 and make all changes in either browser or in SharePoint Designer 2010. Host OS is Windows 7 Enterprise.