This is already possible - you can customise the sub menus on a nav bar to be whatever colour you want. It's just grey is the default colour, expecting that you will change the colours to suit your design
Double click on the nav bar in Web Designer to open the Nav Bar Properties dialog and click Menu Styles.
This dialog changes the colours for ONLY the sub menus, so customise the colours, apply borders, spaceing, transparency and all other settings for them here.
Click OK to both open dialogs and preview the page.
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