Since updating site to 1.8 menu item that has a submenu is displaying with a yellow background.
Menus has the following CSS to display yellow bar above menu items on hover which works on all menu items except the menu item that has submenu.
The ::before is used to be able to move the dropdown via transform, but still keep it accessible. Otherwise, you would always have a gap in between, which we prevent with ::before.
This conflicts with your custom CSS, but is very easy to solve with David’s hint.