Really simple guide 🙂
And I hope it helps everyone out there!

Link to the theme I was using from XDA:
http://forum.xda-developers.com/showthread.php?t=1000049

Other themes you might like:
http://forum.xda-developers.com/showthread.php?t=953450

Written guide for you guys (written by me)
-Download .zip file (do not extract)
-Go unto your phone and tick USB debugging mode, under development, which in turn is under applications.
-(install samsung USB drivers, if not done already, was on my gf’s PC that’s why i had to), Plug in SGS
-Enable usb debugging mode
-Go into INT MEMORY
-Paste .zip file that you downloaded in there
-Turn off and go into recovery: VOL UP+POWER+HOME KEY, or using ROM manager, boot into recovery (You can also make a backup just in case, using this app)
-Now you are in recovery
-Using your VOL UP/DOWN keys go to where it says CLEAR CACHE, select it (Using your power/home key), then scroll down and confirm YES.
(DO NOT WIPE ALL DATA, not that one)
-Then go into ADVANCED, by scrolling down using the VOL DOWN/UP keys, and make ur selection using the POWER/HOME key
-In advanced you will see clear DALVIK
-Select it, then scroll down to confirm YES
-The go back into the MAIN menu, where at the top of the menu you can see “install zip from sdcard”
-Select it
-then scroll 1 down to “choose zip from sdcard”
-Scroll up/down to get to “crysis_revolution.v1r61.zip”
-Click the power button and wait
-Your phone should now reboot (if it doesn’t then just go to the main menu, and select “reboot now”)
-Your phone has now rebooted, wait for all the shiz to load at the lock screen, and then enjoy the theme!

Enjoy!
–TotallydubbedHD