Hi Julio Angel
You need to create the new snippets and paste the link code here and save the code.
And please add the below CSS code in the Additional CSS box. Go to the Appearance => Customize => Additional CSS then paste the CSS here and click to Publish button.
.theme-testimonial-block .thumbnail .rounded-circle {
border-radius: 2% !important;
}
Let me know if it works.
Thanks