If you are on Android and woke up to the brighter, default Gmail look, you are not alone. Gmail's Dark Mode feature has reportedly vanished for the mobile app users.
According to 9to5 Google, the dark mode setting is also missing from the preferences. Several users have reported about Gmail going back to the default white mode.
It is not clear what is causing the dark mode to disappear. However, if you are one of the largely impacted Pixel 2, 3, and 4 owners, you could try to 'Force Stop' the app, reboot the device, or just wait for Gmail to issue a fix.