Paul Brown over kotaku developed a new iMario theme for the iPhone 4 bringing the entire mushroom kingdom on your springboard. Being an unofficial Mario theme you need to jailbreak your iPhone 4 first.
How to install iMario theme on iPhone 4
Step 1: Jailbreak your iPhone 4 (Guide Posted here)
Step 2: Download and install WinterBoard from Cydia.
Step 3: Reboot your iPhone 4.
Step 4: Download iMario theme and extract it
Step 5: Copy its contents to the Winterboard directory (private/var/stash/theme.xxxx) of your iPhone 4
Step 6: Navigate to Winterboard app and select the iMario theme you have copied.
Voila! You have successfully installed iMario on iPhone 4 theme.
Download iMario theme for iPhone 4
iMario for iPhone 4 | Backgrounder
Via – Kotaku