If you manage to find a theme thats not included on bb apps world, follow these simple instruction:
1. First download the new theme to your PC and unzip it if necessary.
2. Plug your Blackberry to your computer and open your Desktop Manager software.
3. Select Application Loader. Choose 'Open', then 'Add'
4. . When the dialog box opens up, find the 'alx' file associated with the theme you have just downloaded, select it, and click OK.
Once your bb reboot, enjoy your new theme. ^_^V