Here are some screen shots. First is side-by-side of (left) how button appears with width, height, font-size controlled by one measure in your customization panel and (right) the tweak I made by reducing the font-size (only needed on small screens).

Side-by-side of customization intent.

This image shows that your CSS is overwriting the custom CSS I tried to add in the CSS customization area provided by WordPress Theme. See that “font-size:1.9em” has strikethrough.

My code of type-size:1.9em is overwritten by your font-size:36px set by 'Icon Size (px)'.

Thanks, Kristi