Go in to the editor in your wp-admin panel and find the code on the style sheet
.my-head {
margin:10px auto 0 auto;
max-width:980px;
padding:0;
background:#ffffff;
border-top:2px solid #efefef;
border-bottom:1px solid #cdcdcd;
box-shadow: 0 3px 3px #dedede;
-moz-box-shadow: 0 3px 3px #dedede;
-webkit-box-shadow: 0 3px 3px #dedede;
-o-box-shadow: 0 3px 3px #dedede;
}
Replace the background color in the style sheet ( #ffffff ) to the color you desire.
You can find any color you want at http://www.w3schools.com/tags/ref_colorpicker.asp
One should always back up their database before an upgrade or theme change using a plugin or manually, as esmi stated above.
If a backup of your website is not done correctly before you update or change themes, files can be lost.
Information on backing up your websites files can be found in the WordPress Codex.
http://codex.wordpress.org/WordPress_Backups
