Open CSS file in your favorite editor. Feel free to edit.

CSS file style.css is located according to the FolderStyle property
Example: ComboBox1.FolderStyle = "combobox/styles/black_glass";

You can edit the existing styles or create new ones.

Background for selected item

CSS:

 .ob_iCboICBC li.ih, .ob_iCboICBC li.ih b, .ob_iCboICBC li.ih i 
{
     background-image: none !important;
     background-color: #CEF0F9 !important;
}