Go to Settings → General → Appearance Select "Dark Mode" or "Automatic" if you want it to change depending on the time.
Set your Apple TV to dark mode
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Boot up the screensaver on your Apple TV
Boot up the screensaver on your Apple TV by double-clicking the menu button on the home screen.
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8 Apple TV Tips and Settings
How to customize Apple TV remote, pair AirPods, calibrate display, and set up screensavers