Whenever anything goes weird with your theme go to the live demo site for the theme. Every blog wearing the same theme here is using the same underlying template, which only Staff can edit. Every edit they make affects all blogs wearing the same theme.
I tagged this thread for Support Staff attention because I believe Themes Staff maybe making changes to the theme template that are causing this issue. If not there’s a bug and they are the ones who must fix it.
I was using the Blue & Black colour scheme and ended up with blue text on a blue background without making any recent theme/colour changes. Now I’ve switched back to the default Blue theme as suggested so at least you can read the headings in the feature slider!