True dark mode will arrive in Android 10. However Android 9 has partial dark mode(s). Here are the 2 places to switch it on - enable both for maximum coverage:
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Settings > Display > Advanced > Device theme. Select Dark.
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Enable Developer mode (Settings > System > About. Tap Build 7 times).
2.1 Search for “night” in Settings.
2.2 Set it to “Always on”.