change fonts & fontsize menu

Hi all
just installed this theme but please help me, I’m about to flipout. I’m trying to change the fontsize of my main menu in the top. I tried everything in the custom css option, by changing the CSS-class and change the settings via that option, but can’t change it.
I hope someone here has the answer for me. Thanks thanks thanks!

Link: https://www.travel-cheaper.nl –> trying to change the size of the menu with “aanvraag” “informatie” and so on