How do I reduce borders on mobile?

Hi I have CSS on my Brine site to increase the border width as I like chunky borders.  However the borders are too big for mobile.  Is there a way to reduce them for mobile only?

The code I'm using is :

#grid {
  margin-right:50px; 
 }
 #grid .item {
   box-sizing:border-box;
   -moz-box-sizing:border-box;
   -webkit-box-sizing:border-box;
   border-top:50px solid #fff;
   border-left:51px solid #fff;
 }
 

mylesformby.com

@milowindsor you can use media query for mobile

/* CSS for only Mobile */
@media screen and (max-width:640px) {
paste your CSS here
}