Yes just look for and change the background for this (around line 324) –
#nav {
background-image: url(images/nav-bg.png);
background-position: center top;
background-repeat: repeat-x;
clear: both;
color: #fff;
font-family: 'Oswald', arial, serif;
margin: 0 auto;
overflow: hidden;
width: auto;
}
Most developers use a text editor program (linked to an FTP program) that shows all of the code by line. You can search for the best html/css text editor depending on which OS you’re using (Mac, Linux, or PC)……and there should be plenty of tutorials out there that can help you set it up.