Yes just look for and change the background for this (around line 324) –
background-position: center top;
font-family: 'Oswald', arial, serif;
margin: 0 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.