Mobile version text too big

Site URL: http://www.4cornersgames.com

Hello,

I'm currently populating my website www.4cornersgames.com and while the desktop version and tablet version (somewhat) look good, the phone version doesn't scale the text down.  If i try to change the text size for the phone it changes it for all 3.  Is there any way I can keep the desktop version text size the same size, scale it down a little for tablet, and scale it down much more for phones? Thank you in advance for the help!

@media screen and (max-width: 639px) {
  p {font-size: 12px}
  h1 {font-size: 20px}
  h2 {font-size: 18px}
  h3 {font-size: 16px}
  h4 {font-size: 14px}
}

Set the sizes as needed.