Dark mode is available on MetaMask Mobile v4.3.0 and later, and v10.13.0 and later in MetaMask Extension.
To turn on dark mode, head to Settings -> General, scroll until you see 'Theme', and then select 'Dark' from the dropdown menu. If this is the first time you access the dark mode menu, the setting will be 'Default', which means the classic light MetaMask theme. Once you've interacted with it, however, you'll be able to pick from Dark, Light, or System.
Dark mode on MetaMask Mobile aligns with your device settings. This means you won't find the toggle in the MetaMask app itself. To change your device settings, see the below iOS and Android guides:
Dark mode on Xiaomi devices
MetaMask's dark mode may not work on Xiaomi devices due to dark mode settings on MIUI OS (a customized version of Android) overwriting our app.
A workaround is to go to Settings -> Display -> More Dark Mode options and then turn off the toggle next to MetaMask.