Lumen

Lumen is a menu bar application for macOS that magically sets the screen brightness based on your screen contents. You control Lumen simply by using your brightness keys - it'll automatically learn your preferences.

Lumen will automatically brighten the screen when you're looking at a dark window (for example, a full-screen terminal) and it'll automatically darken the screen when you're looking at a bright window (for example, a web browser). This makes for a much more pleasant experience, especially at night.