Profile Themes will no longer be available on StumbleUpon as of October 24, 2011. HQ screengrab Picture here:

According to StumbleUpon; In order to continue to deliver the best possible discovery experience, we’ve made the decision to scale back features that are used by only a small percentage of our users, but that require a lot of resources for us to maintain. As such, themes are part of the site features that will be removed.

When are Themes going to be removed?

Themes will be accessible until October 24, 2011, after which your profile will be switched to the default white theme (white background with black font.)