OnePlus
Set Dark or Light Theme for the Messages App auf OnePlus
The Messages app on OnePlus phones has its own theme control that is separate from the phone-wide dark mode toggle. That is why some users see a dark system interface everywhere except inside Messages, or the opposite: a dark Messages app while the rest of the phone stays bright.
This app-level setting exists because Calls & Messages is a preinstalled OnePlus app with its own display preferences. Choosing System default links the app to whatever theme the phone itself is currently using, including any scheduled day/night switch. Choosing Light or Dark manually overrides that and keeps the app fixed to one look, no matter what the rest of the system does.
To change it, follow this path: Open the Messages app ➔ Open the menu ➔ Tap on Choose theme ➔ Choose Light, Dark or System default ➔ Confirm with OK.
Pick Dark if you mainly use the app at night or want to reduce power draw on an AMOLED panel. Pick Light if you find the dark layout harder to read text on in bright rooms. System default is the better choice if you already rely on a scheduled dark mode and want every app to match it automatically. If the app still does not change after confirming, a restart of the Messages app usually applies the setting correctly.