Making a menubar… general steps

I’d like to make a menubar for my site (a live beta can be found here): https://www.dogsoverlava.com/lunatec

I’m using the “benevolence” template. I’d like to make something like the menubar found here: https://www.arcadefire.net (also using benevolence template).

Now I can see the style.css of the arcadefire site and can identify the relevant css. Am I wrong to think that I can cut and paste those portions (with some minor changes) into my own styles.css?

Now I’m not sure what to do next. I thought I’d have to create a php file called “mastmenu.php”…. or something similar. Also, don’t I have to somehow “call” this file into play om the main template?

Can you guys give me some guidance? What are the general steps ?