Since this keeps coming up, I'm going to post the FAQ article I just did up here as well. I'd say this should be stickied.
There are several reasons why an installed theme, which is visible in Options->Advanced Options->Applications, will not show up in Options->Themes:
- Dimension Themes are not installed: In order to install custom themes, the Dimension theme of that style must be on the device (ie, Today themes require Dimension Today, Zen themes require Dimension Zen, etc). These themes are on all devices by default, however some memory guides recommend removing them to save space.
- Theme was not build for your device/software version: The theme must have been build for your device series (ie 83xx, 88xx, 81xx) and your current, or an earlier software version (ie a 4.2.2 theme will work in 4.3.0 and 4.5.0, but not 4.2.1)
- Battery was not pulled after installation: For themes on devices running software before 4.2.2, the battery must be pulled to complete installation
- IT Policy restriction: If you are on a Corporate BES which restricts installation of third party applications, you will not be able to use custom themes
- Not enough Free Memory: In rare cases, to little memory can prevent a large theme from being activated.