Responsive columns

Hello,

I’ve created three columns within a row and set their width as 33.3%.
I’d like to make them responsive and have a width of 100% on mobile.

This is my media query style:

@media (max-width: 780px) {
 .panel-grid-cell {
  width:100%!important;
  float:left!important;
 }
}

Unfortunately it doesn’t work.

Any help?

Thanks