Use the following syntax for doing media queries, if possible.
```
.my-class {

  // Mobile styles by default

   @media (max-width: @width-medium) {
      // Your styles up to @width-medium
   }
   @media (max-width: @width-large) {
      // Your styles up to @width-large
   }
} 
```
Refer to https://github.com/mozilla/webmaker.org/blob/master/public/css/breakpoints.less for a list of breakpoints you can use