Firstly, to use any themes or modded apps, it is required to be JAILBROKEN ! You can read my tutorial about it here.
⛔ I’m not Iphone user anymore but i used Electra jailbreak on iphone 6s plus on iOS 11.3.1 for my mods, so i can’t guarantee if it works or not for any other iOS versions so before making any change on your iphone, ALWAYS BACK UP YOUR ORIGINAL FILES ⛔