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CSS on Flyout Menu; Need Border

Hello. I need a little CSS help here.

I have found that adding a border to .sub-menu will add a block box around my flyout menu but it also adds a black box around the menu that appears in the main menu (to the left) when I'm on a sub menu page.

I am trying to add just a black box around the flyout menu ONLY and not on the main menu to the left.

Any help here?

Thanks in advance,
David