Yes many manufacturer use their own skins. It is possible to revert if phone is rooted. You have to make a custom ROM for your phone by editing all the themed items to stock one, but this requires skills. I tried changing theme by modifying a ROM and its not easy. at least not when you are beginner. Another easy way is to flash already made custom ROMs. Pure/stock ROMs are called AOSP ROMs or AOSP based ROMs will just do fine.