Trouble customizing menu/header on one of the the Site Library templates

I installed GP Premium on an eCommerce site I'm going to be building and imported the "Flint Skin Merch" template out of the library (See here - https://generatepress.com/site-library/#). I'm in the early stages of customizing it, but having some issues with the main navigation or header (the main menu at the top).

I want to change the colours and customize it to be the same across the entire site (every main page, product pages, blog and custom posts) but am struggling to figure out how to do so. I was able to get my customizations set up to display on certain pages, but it wouldn't seem to work on the homepage. Anyways, if it's custom CSS or whatever else it may be, I was hoping someone might be able to walk me through how to change that.

I would really appreciate it, thank you!!