How do I apply custom CSS to a specific testimonial widget

The easiest thing is to check the source code of your page with the widget and look for the testimonial widgets div container id tag. It’ll be something likeid="testimonials_widget-3".

Then, in your theme’s custom.css or styles.css file write CSS like the following.

#testimonials_widget-3 {
    color: red;
}

#testimonials_widget-3 cite {
    color: black;
}