Xiaomi
Dark or Light Theme for Messages App auf Xiaomi
The Messages app on Xiaomi phones keeps its own theme setting, separate from the system-wide display mode. This is why switching your phone to dark mode does not always change the background of Messages, Calls or Contacts at the same time.
Each of these apps stores its own preference inside the app menu. If Messages still opens in a bright white layout while everything else on your phone is dark, the app simply has not been switched over yet.
Where to change it
Open the app and tap Tap on Messages, then go to Open the Menu and select Tap on Choose theme. Pick Choose Light, Dark or System default and tap OK to apply your choice.
Setting the option to system default keeps Messages in sync with your phone-wide dark or light mode automatically, so you only need to change the system setting once afterward. Choosing Light or Dark directly overrides that behavior and keeps the app fixed in your selected look, even if the rest of the phone switches.
If the theme option is missing or grayed out, an app update usually restores it. A forced light background can also point to a battery saver mode overriding display settings temporarily.