Hello,

First of all thanks for choosing our Theme, we are glad to be you in the WoodMart WordPress family :).

Please try adding the following Custom CSS in the Custom CSS for Desktop area under Theme Settings >> Custom CSS.

.menu-item-375.menu-item-design-sized > .sub-menu-dropdown {
    background: rgba(0, 0, 0, 0.61);
}

Regards.
Xtemos Studios.