(Instructions are originally from Lou.)
Before you start, please make sure you have Application names in iFile switched on. To do this open iFile, press the Settings icon in the lower left - press File Manager - switch "Application Names" to on.
It's pretty straight forward as long as you have the basic knowledge of how to use iFile.
Simply download from the link below - Unarchive - Open the blue folder that has now appeared named Fancy Instagram Theme - select ALL images within the folder - cut them to the following location:
User > Applications > Instagram > Library > Application Support > Instagram > assets
These will replace the stock Instagram images. Just close your Instagram app from the switcher (or respring) and the theme should take affect.
If you want the old look back at any time, simply uninstall and reinstall Instagram.