How to add custom font for a testimonials element

install your font. – Give a custom-class to your testimonial ALB ( this custom class will be on the testimonial wrapper container) .
Choose the selector that fits for you.
means decide if you like to have that font only for the avia-testimonial-content or for the whole avia-testimonial_inner etc.

f.e.:

#top .avia-testimonial-wrapper.custom-class .avia-testimonial-markup-entry-content {
  font-family: "brush script mt" !important;   /*** see what your bespoke font is named ***/
}

see ( a different font only for the testimonial ( not meta etc. ): https://webers-testseite.de/testimonial-styling/
The code on that page is only for that specific styling there