A free app for Android, by Gianluca Spadazzi.

Transparent Black – CM13 Theme is a transparent black theme that will make many apps of your phone or tablet black. It is a free, open source, and fully compatible with the latest versions of Android.

This is the best choice if you want to have a clean and simple look at your phone. You can also choose a wallpaper for your home screen and lock screen. The theme will be applied instantly and you can see the changes in your device right away. You can also choose to change the font and color of the apps, which is another way to have a fresh look.