Yes, Dark mode is available in Xperia 1. It's called Dark Theme.
How to enable dark theme
- Go to Settings
- Tap Display & select Advanced
- At the bottom, you will see the Device Theme menu, in which you can choose "Dark" theme
I've activated Dark theme, but nothing happens?
Dark theme doesn't apply to the entirety of the phone. The settings menu won't change colours, nor will any of your apps.
However, some items do benefit from Dark Theme:
- Quick settings bar
- Google Now
- App drawer
- Volume menu
- Power menu