Don't have an answer for you but could you tell us what browser you are using? If I have it installed I'll poke around and see if I can reproduce the problem here. I know there is some issues related to Chrome's latest Dev version and I think maybe some still with IE9 not sure but would be glad to do so experimenting if you like.
2.4 is the new theme and it is the default if you are not logged in. Upon logging in, you should default back to the theme of your choosing. As a guest, you can also choose a theme, but once you log in or quit the session, this information is no longer saved.