When I open Youtube with my account the appearance is defined to use the device theme, but the colors are the ones used in the light theme instead of the dark theme (My device is defined to use the dark theme). In
brave://settings/appearance is selected
Same as macOS. If I change Brave appearance to dark in
brave://settings/appearance Youtube appearance use light theme colors when is selected to use the device theme. On Safari that doesn’t happen, so for me is a problem of Brave browser.
YouTube should use the same device theme and in its definitions is select to use the device theme.
Brave browser version:
Version 1.27.108 Chromium: 92.0.4515.107 (Official Build) (x86_64)