If you’re running macOS Mojave (and you really should be), probably one of the first things you did after installing it was to test Dark Mode. The new theme darkens the Mac interface and makes everything look cooler — apart from text, which is just harder to read.
But there’s no built-in way to have Dark Mode switch on automatically at sundown. That’s where NightOwl comes in. The free app switches your Mac between Dark Mode and the traditional lighter look automatically.