There are four predefined testimonial themes in the plugin, You can select theme from the plugin setting like that default theme, default dark theme, default light theme, and default shadow theme. I will explain to you how to change the theme. Refer the screen shot.
How to change testimonial designs?
Go to => Testimonial => Themes Settings =>
Here you can change the testimonial themes. Simply select the theme then Save Changes. You can also see the preview of your selected theme. Refer the screen shot.