Traditionally, if you want to really customize your Android experience, you have to root your phone and flash a new ROM€”essentially a whole new version of Android€”that has the features you want. If you don't want to go that far, or you only want to tweak specific parts of Android, you have another option: It's called the Xposed framework, and you can use it to customize your phone from top to bottom, all without flashing a new ROM.