Disable Dynamic Menu Bar in macOS Big Sur
Is there a way to prevent macOS from changing the menu bar from light/dark mode whenever a wallpaper changes, and if so, how to do so?
The Accessibility System Preferences give you some control over the menu bar. The "Reduce transparency" check box will remove the transparency in the menu bar so that it does not show the desktop wallpaper.
And it does reduce transparency in other places (e.g. Dock and window backgrounds). If that is a problem then @TJ Luoma has the best answer with Option B.