A new colorization & theming user interface will be a major theme for Android 12 when it is released later after beta testing

Android 12 could get a colorful, new theming system (androidauthority.com)

Android 12: In-depth theming system that can recolor apps – 9to5Google

According to information viewed by 9to5Google, Android 12 is working to bring a native theming system, which allows you to select a primary color and an accent color, and see those colors reflected throughout Android — where supported, more on that in a bit. The primary color chosen should even affect things like the background color of notifications and the Quick Settings area.  What really makes Android 12’s theming system special, however, is that your chosen primary and accent theme colors will also be accessible to developers of Android apps. That means, if their developers so choose, your favorite Android apps can better match your phone’s chosen theme.