Testimonial Slider Responsive CSS Issue

I need help with my testimonial slider responsive CSS, been struggling with it. I have purchased the premium version and would like to keep the slider centered and only show one testimonial at a time. It changes to showing more than one testimonial once you reach a certain width and it loses it center alignment too.

I am using this on two websites, my site is https://www.shimmertechno.com, the slider is on the front page. Only have one testimonial though.