Background image coding that keeps menu color at the top.

Could someone help me code a background image into my theme that doesn’t take over the banner at the top?

I’ve followed a few tutorials and the image either takes over all the colors in the menu or keeps the menu but is too zoomed in!

The image I’m trying to use is just a simple grid pattern I made in illustrator so if there is a way to just build a pattern that could work too!