Twenty Twenty-One: Navigation block has a permanent theme-imposed background color

[kjellr]

Twenty Twenty-One provides a default background color to the navigation block. If the block appears inside of a differently-colored container block, this looks incorrect:

If a user attempts to modify this color via the Navigation block controls, only the menu item backgrounds are updated. The theme-provided background persists:

The theme should leave this block transparent by default, so that it will work on a variety of different backgrounds.

[joyously]

This is a common problem with menus. It needs to use a CSS Custom property, so it can be consistent in the menu, yet still have a background color for the dropdowns if there are any.
Using transparent doesn't work well for dropdowns.
And yet, the text color needs to go with the chosen background color.