When the service first launched, it had started with a dark theme and doesn’t have a light mode yet. The interface was mainly black with shades of gray. However, some of the lighter gray parts are now black with the new redesign.
Android status bar is now darker and 9to5Google you notice that the change is evident on the Library tab and you can see the gray carousel “Recent Activity”. Also, the results on the Browse tab, search and playlist pages are combined at the top.
Although these are small tweaks, they seem to create a more unified viewing experience for users. It seems that the changes started rolling out a few days ago.
Some have suggested that these changes could mean plans to roll out a lightweight theme for YouTube Music, as these tweaks would make the interface investment easier.