Yesterday I have your same issue, theme button didn't work.

What happen to me:
16th I updated the phone with the last firmware, making before a backup via backup app, and It become restart constantly, so I reflashed and I install all the applications via backup, then during the installation of apps the hildon desktop crash and so I have to switch off the phone.

After the restart and reinstall everything I found with your problem, theme button didn't respond.

Yesterday I reflash the phone, I verify with the first run of the phone after the reflash that with factory settings theme button works, so then when I was going to install all applications I deselect any theme I have in the list of applications to install.

After this the Themes Buttons works well.

What could you try?

Try to uninstall all themes you have installed and see if it works and I you didn't have anyone to uninstall, try reflash, but made a backup before, remember!!! or wait if there's somebody that could give you some tip.

This was my solution and it works.