you can also modify the css directly. in the css/common.css file, edit the background-color property of the input[type=submit] selector:
input[type=submit] {
background-color:#2C6190;
border:0;
color:#FFF;
font-family:Arial, Helvetica, sans-serif;
text-align:center;
width:142px;
height:33px;
font-size:12px;
font-weight:550;
}