Editing the background color of nested menus in Tremont (Custom CSS)

Site URL: https://burntpizzacork.ie

I'm using the Tremont theme and am trying to find a way to change the background color of the nested menu.

Currently it's transparent, and this poses a problem because when the dropdown is expanded, the text opens up over other text and isn't readable. I would like to add a background colour and border to the drop down menu.

I'm currently trying to use the following code but it doesn't seem to be working:

.Header-nav-folder{
  background-color: #000000;!important;
  border: 1px #ffffff
}

The site is  https://burntpizzacork.ie

Thanks for your help.

Used this CSS (Design > Custom CSS)

.subnav {
    background-color: rgba(0,0,0,.85);
    border: 1px solid #fff;
    padding-right: 10px;
    margin-top: 10px;
    max-width: 120%;
}