Sub-Menu CSS

Hey guys,
I’m stuck trying to match my header sub-menu background colour to my header colour. I can’t seem to get the CSS right.

If anybody can help, I would be very grateful.

Page – https://www.in2drums.com/wordpress/live-lessons

Main header CSS (Working)
#masthead {
background-color: rgba(10,10,10,0.3);
position: absolute;
z-index: 1!important;
top: 0px;
left: 0;
}

Sub-menu CSS(not working)

.sub-menu {
background-color: rgba(10,10,10,0.3);

}