after pseudo-element in menu

Hello.
I'm trying to add a content after the main menu items but I've got something like this https://prnt.sc/gzvpyr instead of something like this https://prnt.sc/gzvq6h
I always use this css code

#mymenu > li{
	display: inline-block;
}
#mymenu > li:after{
	content: "-";
}

How could I achieve that using GeneratePress?

Thanks!