Brine 7.0 Slideshow Gallery - Need help making arrows smaller on mobile view

Site URL: https://american-paving-design.squarespace.com/

Hi there,

I can't figure out how to make the left and right arrows in the screenshot below smaller. The gallery size and caption size is fine, it's just the grey arrows that I would like to make smaller on mobile view only. 

Thanks in advance for any tips. 

Add to Design > Custom CSS

@media screen and (max-width:640px) {
.tweak-index-gallery-controls-icon-weight-heavy .Index-gallery-control .Icon {
    width: 15px !important;
}
}