Android
Dark or Light Theme for Messages App auf Android
Google Messages has its own theme setting that runs independently from the phone-wide dark mode toggle. That is why switching system dark mode on or off does not always change how Messages looks. The app only follows the system automatically when it is set to System default.
To change the behavior, open Messages and tap your Google Account icon. Go to Messages settings ➔ General ➔ Choose theme, then select Light, Dark or System default and confirm with OK.
System default keeps Messages in sync with the rest of your phone. Choosing Light or Dark manually locks the app to that appearance, even if you switch the system theme later. This is useful if you want Messages to stay dark for battery reasons on an OLED display, or stay light for readability regardless of your general phone settings.
The setting only changes the visual appearance. No messages, contacts or chat history are affected. If the entire phone, not just Messages, refuses to switch between light and dark, check the system-wide dark mode setting instead of this app-level option.