You need to use the background-color selector to change your Tumblr theme's background color.
This selector will be in the body section of the css (lines 31 to 38):
<style>
body {
margin: 0;
padding: 20px;
background-color: green;
font-family: arial;
font-size: 0.75em;
color: blue;
}
You want to change the italicized word in the bold line. You can write in a hexadecimal number for the color (see here for more information on hex) or you can write a different color word, such as purple or red instead of green.