Use this code...
#g-navigation .g-main-nav .g-sublevel > li > .g-menu-item-container {
font-size: 0.9rem;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.
The following users have thanked you: Oliver