Might I add another solution to a rare case.
Depending on the way your device is set-up, you may have application permissions by default set to deny. If this is the case you may sometimes have to change them; example scenario being a theme not showing up after install.
You would have to go to Options>Advanced Options>Applications, move down onto the theme and click. On the resulting menu click "Edit Permissions". On that screen go over all of which in this case would be set to deny, and click and select "Allow" for all.
Afterwards do a battery pull or reset and the theme should show up.