DropDown folder background colour

Site URL: https://eisjmun.org/

When hovering over the tab "conference", there is a dropdown that cannot be seen due to the banner. How do I change the background colour of it to white so it can be seen over the banner? I tried the css:

.header-nav-folder-content (
   background: #febf00 !important)

with no success.

The dropdown has currently been removed from the site as we are an active company with users, but will be brought back when a solution is present
}

Use CSS

.subnav {
    background-color: rgba(0,0,0,.3) !important;
    z-index: 50000 !important;
}