Changing button font size on mobile

Site URL: https://www.bijouxcreative.com/branding

I am trying to change the font size on my medium block button for mobile only.

Does anyone know the correct code please?

https://www.bijouxcreative.com/branding

Add to Design > Custom CSS

/* medium button block size mobile */
@media screen and (max-width:640px) {
a.sqs-block-button-element--medium.sqs-block-button-element {
    font-size: 10px;
}
}