Hi!
The background color is coming from the widget container. The widget container layout is purely controlled by the theme CSS. However you should try the following custom CSS:
[html]*[id*=ajaxsearchprowidget] h3.widget-title {
display: none !important;
}
*[id*=ajaxsearchprowidget] {
background: transparent !important;
}[/html]
These will force to remove the background from the widget container.