Hello Lily – Testimonial

Click on the testimonial block.  Then make sure you are on the parent block settings for it, not an individual testimonial.  So in the bar at the top, you will click the left 3 stars and it will turn into a square with circle, meaning you are editing the parent settings.  You will see that circled in the screenshot.  To add a testimonial click the + icon. 
To change the fonts you want to be on the parent block settings, not the individual testimonial settings.  Then in the editor to the right, under the style tab you can open each section and adjust the fonts and colors.