A free app for Android, by BIG Launcher.

Make your phone sleek and sassy with BL Monochrome Dark Theme. The app’s dark theme makes phone usage easier on the eyes. Tap “Preferences,” “Appearance,” and “Select Theme” to apply the theme. The tool also offers custom icons for your apps for maximum personalization. To do that, tap “Preferences” and “Customize buttons and screens.” From there, you can select any icon you like. 

Although the application only requires Android 2.1 and up, avoid installing it on low-end devices as it’s memory intensive. Also, bear in mind that BL Monochrome Dark Theme needs BIG Launcher or BIG Launcher FREE DEMO to work.