One of the first things I do when I buy a new device is figure out how to get it rooted. Once I'm rooted I'll figure out how to get my bootloader unlocked and twrp installed so i can flash a custom rom. One of top reasons I started to use custom roms was the ability to change the entire UI with a custom theme. Today OEMs are allowing users to install custom themes in the stock UI without the need to root. Some top flagships like the Galaxy S6 and M9 have their own theme stores. The Asus Zenfone 2 also has an option to change the theme. To apply the themes long press the home screen, find the middle row and choose the themes option, tap on the option you would like to choose, choose your desired default options and you are done. You can also download custom app icons in the Play Store which will show up in your themeing options.