Conditional CSS in GWT app?

I have some conditional css in my project:

@if (com.company.shared.util.PlatformUtil.isSkinny()) {
     .mainHeaderBody {
          width:98%;
          margin-left: auto;
          margin-right: auto;
     }
} @else {
     .mainHeaderBody {
          width:90%;
          margin-left: auto;
          margin-right: auto;
     }
}
 

and some other more complex stuff.. but even this simple case get's quite a bit of red highlighting about missing {'s and }'s

Is this GWT feature supposed to be supported in 10.5.1 or do I just need to live with it for a bit?