This will depend on the pages with or without a banner as the menu is coloured differently for each. For pages that don’t have a banner image (or slider) on, go to Appearance >> Customize >> Colors
Look for the “Submenu Background No Banner” colour setting. This will the one you want.
But, for pages that have a banner image (or slider), the dropdown has a transparent background colour but there is no colour setting because the WordPress customizer doesn’t have an RGBA colour control option. So we need to do this with some custom CSS:
@media (min-width: 992px) {
.main-navigation ul ul {
background-color: rgba(0,0,0,0.30);
}
}
To add custom CSS, I recommend using a plugin like “Simple Custom CSS”, or, if you are using Jetpack, you can use it’s own Custom CSS feature.
Basically, copy and paste the above CSS code and paste it into your custom stylesheet that the plugin will create. The background-color you see in the above code is for the colour of black and has a transparent value of 0.30 (which means 30% transparency).
You can use an online colour picker like this:
RGB Colour Tool
