example.less

selector {
  .size(30px, 40px); /* set default */
  .size('tablet', 25px, 35px);
  .size('mobile', 20px, 30px);
  
  .position(100px, 40px);
  .position('tablet', 120px, 50px);
  .position('mobile', 130px, 60px);
}

ress.less

.position(@x: ~'', @y: ~'') {
  left : @x;
  top : @y;
}
.position(@mode, @x, @y){
  @media screen and (max-width: @@mode){
    left : @x;
    left : @y;
  }
}
.size(@w: ~'', @h: ~'') {
  width : @w;
  height : @h;
}
.size(@mode, @w, @h){
  @media screen and (max-width: @@mode){
    width : @w;
    height : @h;
  }
}

setup.js

var configs = {
  presets : {
    mobile : 640,
    tablet : 1024
  }
};
var current_mode = '';

var setup = function(){
  less.modifyVars(config.presets);
}();

$(window).resize(function(){
  var w = $(window).width();
  
  current_mode = 'default';
  
  for(i=0;i<config.presets.length;i++){
    var preset = config.presets[i];
    
    if(w < preset.max_width)
      current_mode = preset.name;
  }
  
  less.modifyVars({
    '@mode' : current_mode
  });
}());