If you're having trouble – make sure to click "On" in the top left rather than the switch in the top right once it opens System UI Tuner.
Note: On Android 7.1 and higher – this feature is officially built into the operating system, called Night Light. However, this feature requires a driver, and as a result, only is available on Pixel and Pixel XL (and newer devices). As a result this will not work on Nexus devices on that version.
For some reason people demanded this – but now there are in-app purchases.
Night Mode Enabler