How to edit .mh-header-nav

Hi, I’m using MH Magazine Theme and I’ve been trying to customize the header menu.I’m trying to find the right css code to edit the menu items, font-size etc. and i’m trying to change the totalt width of the menu bar (see image, I want the menu items to stretch all the way from left to right, like on the main nav below.).

You’ll find my site at: https://www.filmtopp.se.
Can you help me? I’m going nuts over here…