In the first beta, the dark theme switch was removed, but it is possible to activate it via ADB. To do this, it’s enough to type simple commands in the terminal (you need to install ADB and activate the “debug mode” on the device):
adb shell settings put secure ui_night_mode 2 - Enable
adb shell settings put secure ui_night_mode 1 - Turn off
adb shell settings put secure ui_night_mode 0 - Automatic