Are you browsing the site using a phone? I have a mod installed that detects when a phone is using the site, which makes the mobile style the default. That's odd that you couldn't change the theme at all though. Did you try through your profile settings in addition to the style changer?
If you're talking about using the style changer on the lower right hand corner though, I noticed it doesn't seem to always work unless you're on the home page. If you only tried that, make sure you're on the homepage when using it if it isn't working for you.