Below is a working example of what you want.
http://alt-web.com/FluidGrid/Fluid1.html
In my custom Content.css file, I have some divs set to display:none wrapped inside a mobile media query like this.
/* Special Rules for Mobiles */
@media only screen and (max-width: 481px) {
/**hide some divs**/
#LayoutDiv5, #LayoutDiv6, #LayoutDiv7 { display: none }
}
Nancy O.