The dark mode option is locked behind a feature flag, so you'll have to enable it.
- Open Chrome and navigate to
chrome://flags, then search for "dark".
- From the list, switch Dark/light mode of system UI and WebUI dark mode to
- Restart your chromebook to apply the changes.
Now that the option is available, you just have to switch it on!
- Open the Settings app.
- Navigate to Personalization, then click Dark mode.
- Switch the option on!