Order confirmation title on mobile

Site URL: https://www.at17c.com/

Hi. 
When you get the "in-browser" order confirmation page on mobile the title is way too large. 
How do I reduce that size? Is there a way to preview this without having a "free" product on my page. 
Screenshot of how it looks when ordering. 
TIA

Add to Design > Custom CSS

@media screen and (max-width:767px) {
#confirmed-page h1 {
    font-size: 20px !important;
}
}