Change the styling on drop down menus/ folders in 7.1

Using 7.1, is there a way of editing the style of the drop-down menus in navigation? Maybe just change the background colour?

my site is carbonbuddyproject.org

Thanks, Emma.

https://www.youtube.com/watch?v=yzK_fJc9W3w
this is also helped me to change it.

It's beyond me why this is not one of the 100 options in the default site styles when you edit each theme... 🤦‍♀️