Adding Background Image / Wallpaper to Entire Site or Header?

Site URL: https://www.buffalobiketours.com/

Hi, I'm trying to figure out how to add a background image as wallpaper for my entire site. Not wanting a gradient, but rather a repeating image. I'm using a Brine template in 7.0. Any tips? It's greatly appreciated 🙂

Add to Home > Design > Custom CSS

.Main {
    background-image: url(https://beaverhero.com/wp-content/uploads/2019/07/coffee-beans-1082116_640-min.jpg);
    background-size: cover;
    background-position: center center;
    background-repeat: no-repeat;
}