The Desktop Method
One method for adding or removing BlackBerry themes involves both your desktop or laptop computer and your BlackBerry device. In order to utilize this method you will need to have the BlackBerry Desktop Manager software installed on your computer and have your BlackBerry device successfully connected and recognized by the computer.
If you do not have the BlackBerry Desktop Manager software installed you may find the official executable here.
You must also have the theme file downloaded to your computer in order to add the theme using the desktop manager software; so make sure the file is on your computer or redownload the file by using your computer to find it again on an internet browser.
After making sure your device is connected and recognized by your PC, start up the BlackBerry Desktop Manager software (if it does not run automatically) and then wait for the menu to load.
Once the BBerry Software has loaded you must then select the "Application Loader" option. Then from the next menu select the "start" button underneath the "Add/Remove Applications" section.
The program should then reconnect to your BlackBerry phone and populate a list with current applications and programs that are installed on your BlackBerry device.
Adding Themes: Once the program has refreshed properly you must select the "browse" button and navigate to the location on your computer where the theme is stored. Select the theme file (it must be in .alx format and must include a .cod file along with it) and wait for the program to return to the application loader screen. The theme you just selected should be highlighted in the list and the description under the action column should say "install". To continue click the "next" button and wait for the list to refresh; it will provide you with a list of current programs/themes that are going to install on your device during the operation. After making sure the correct theme you would like is chosen, simply click the "finish" button to continue and allow the program to install the theme to your device; once finished you may disconnect your phone.
Removing Themes: Once the program has refreshed properly simply navigate through the list to the theme you would like to remove and uncheck the box next to it (the box should then be empty). After selecting the theme press the "next" button and continue.
The program will then provide you with a list of the themes/applications you chose to remove from the device. Simply check to make sure you selected the correct theme and then click the "finish" button. The program should then do its thing and remove the theme completely from your device.
I would recommend soft resetting your phone after the entire process has finished.