put the class on the menu item here “CSS Classes (optional)”. This will be applied on
[Code that wrecked the forums fixed]
If you can’t see the “CSS Classes (optional)” in the menu item, then there is “Screen Option” in the right top of the screen and there are the options “CSS Class” under “Show advanced menu properties”.
CSS should generally be modified using a child theme or custom CSS theme option or add via a plugin if your theme does not have the option. If you change theme files, your changes will be lost when the theme is updated.