What problem does this address?

The Navigation block, by default, is transparent, and submenus are white with black text:

When you define colors on the navigation block, submenus are able to inherit those, and look the same:

What I'm unable to do, is colorize just the submenu, and have the main menu stay transparent, like this:

What is your proposed solution?

Allow setting the colors of main and submenus individually:

Note: Fixing this ticket also fixes #30165.