Pre-Requisites : Your Android should be rooted: How to Root your Android mobile.
You can get themes from Here.
Step 1. Rename the theme file to update.zip and copy the theme file (update.zip) , to the root of your SD card.
Note: Step 2 to 4 are optional and only for your safety (by creating backups)
Step 2. Now you need to take a backup of your Android phone , in case you wish to restore the phone to its original state. Reboot your phone in Recovery Mode , by following these steps :
* Press and hold the Power/End Call button.
* When the phone is switched off , press and hold the Power and Home buttons. Continue to hold these buttons , until the Android image or logo appears.
* Release the buttons and wait for the Recovery screen.
Step 3. When the Recovery Screen appears , scroll to and select Android vX.X ( here X.X is the version of Android , running on your phone ) . You can alternatively press ALT+B.
Step 4. Wait for a few minutes , until your phone displays “backup complete” on the screen.
Step 5. Now you can apply the theme safely to your phone. Scroll to and select update.zip , or press ALT+A. Press the Home button to confirm your theme selection. Wait until the screen says “install from sdcard complete”.
Step 6. Finally scroll and select Reboot Now or press Home+Back.
It may take around 5-10 minutes for your phone to reboot with the new theme.