Hello,

The background is controlled from:

.main_dnnmenu_submenu

eg, taking the default DNN blue menu code and changing the background from blue to pink:

.main_dnnmenu_submenu {
    background-color: pink; 
    z-index: 1000;
    cursor: pointer;
    cursor: hand;
    filter:progidXImageTransform.Microsoft.Shadow(color='#696969', Direction=135, Strength=3);
    padding: 1px 1px 1px 1px;   /*explicitly set padding for sub-menus */
}


You can analyse the css ids and classes used by using the web developers toolbar and the 'Information ' / 'Display Element Information' option - just click  on an element in your menu and it will display the ids and class details.

further information on this is available here: Troubleshooting DotNetNuke Skins