Background color of a single widget

Hello,
is it possible to change the background of a single widget? I only found in another topic

.sidebar .widget {
background-color: #333 !important;
color:  #fff;
}

to change all sidebar area, but I’d like to do that for a single widget (I have the widget id but I can’t integrate it in CSS to make it work)
I need to put a contact form in the sidebar widget (I did it with magic wand, but I’d like to set a different background for the widget, to get more prominence.
Thank you