Modify the Menu CSS for Upfront Fixer theme

I am not a novice when it comes to CSS, but I am having a hard time getting my head wrapped around how to modify the Menu for the fixer theme. I have made minor changes, but for the hover, sub menu's and the responsive menu's have me confused..

Is there a short tutorial on how to modify the css for the menus.

Everything else is pretty straight forward but this menu css drives me crazy.