The background color of the submenu items is determined by these styles:
.MainMenu_SubMenu {
Margin:0px;
background-color: #e74d52;
border-top: #1e3647 1px solid;
border-right: #1e3647 1px solid;
border-left: #1e3647 1px solid;
border-bottom: #1e3647 1px solid;
/*filter:progid
XImageTransform.Microsoft.Shadow(color=#FFFFFF, Direction=180, Strength=0);
-moz-opacity:.9;
filter:alpha(opacity=90);*/
padding:0px; font-weight: normal;
}
.dnn_dnnsolpartmenu_ctldnnsolpartmenu_spmsub .MainMenu_MenuItemSel {
background-color:#e74d52;
cursor: pointer;
color:#ffffff;
text-decoration:underline;
font-family: Tahoma,Arial, Helvetica, sans-serif;
font-size: 11px!important;
font-weight: normal;
font-style: normal;
border-bottom: #fa8389 1px solid;
border-top: #ffffff 0px solid;
padding:3px 5px;
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You don't have a separate style for the sub-submenu items.
I really would recommend that you switch to the DNN Nav Menu as it is much easier to style it. Take a look at these tutorials.