I can't get the background to stay changed

I few months ago I changed the background to Simply Dark. Now I'm tried of it, and I've been trying to change it to Holiday, or anything else! When I pick the theme from the dropdown list, the page changes. BUT, when I click on anything else, the page changes back.

Help!
Thanks!

Try clearing out your cookies from your browser and then give it a shot :)

It worked!! Thank you!!!!!!!!!!!!!