Example to set widget gradient background color

See What’s NNN of “.testimonials-widget-testimonialsNNN”? for NNN identification. Then write CSS like the following change the color behind the widget to a light gradient gold.

.testimonials-widget-testimonials3 {
    background-image: linear-gradient(to bottom right, #FFEF87 0%, #EFEFEF 100%);
}

A special thanks to inode86 for this suggestion.