Here it is the tutorial for installing OTA ZIP file theme to your blackberry curve 8520..
A. Download from Your PC
1. Download your new theme to PC
2. Extract the ZIP File. Press the right-click in the theme ZIP File. And then choose 'extract to'
3. Connect your Blackberry to PC
4. Copy Folder to Media Card
5. Unplug your Blackberry from your PC
6. Then, on your Blackberry, choose Application - Files - File Folders - Media Card - "(your folder theme download)"
7. Open the Folder and choose file .jad to install your new theme
8. Enjoy your new theme ! \(´▽`)/ \(´▽`)/ \(´▽`)/
B. Download from Your Blackberry
Download your new theme to your Blackberry
2. Open your Media Card
3. Extract "File" ZIP using FILE SCOUT application
4. Open the "Your Theme Folder" on your Media Card
5. Choose file .jad to install your new theme
6. Enjoy your new theme ! \(´▽`)/ \(´▽`)/ \(´▽`)/