This plugin extends Less.JS processor for "attach(url)" and "detach(url)" methods, that allows dynamically attach/detach ".less" files to document.

dynamic.less.js

/**
 * Attach LESS file: less.attach(url)
 * Detach LESS file: less.detach(url)
 */
(new function(){
  if (typeof less == "undefined") less = {sheets:[], refresh:function(){}};

  function extractId (href) {
    return href.replace(/^[a-z]+:\/\/?[^\/]+/, '' )  // Remove protocol & domain
               .replace(/^\//,                 '' )  // Remove root /
               .replace(/\?.*$/,               '' )  // Remove query
               .replace(/\.[^\.\/]+$/,         '' )  // Remove file extension
               .replace(/[^\.\w-]+/g,          '-')  // Replace illegal characters
               .replace(/\./g,                 ':'); // Replace dots with colons(for valid id)
  }

  function getId (sheet) {
    var href = sheet.href ? sheet.href.replace(/\?.*$/, '') : '';
    var id = 'less:' + (sheet.title || extractId(href));
    return id;
  }

  function toLink (url) {
    if (typeof url == "string") {
      var link = document.createElement("link");
      link.rel = "stylesheet/less";
      link.href = url;
    }
    return link || url;
  }

  function getIndex (url) {
    var link = toLink(url);
    var index = -1;
    for (var i=0; i<less.sheets.length; i++) {
      if (less.sheets[i].href == link.href || less.sheets[i].title == link.title) {
        index = i;
        break;
      }
    }
    return index;
  }

  function attach (url) {
    var link = toLink(url);
    var index = getIndex(link);
    if (index < 0) {
      less.sheets.push(link);
      less.refresh();
    }
    return link;
  }

  function detach (url) {
    var link = toLink(url);
    var index = getIndex(link);
    if (index >= 0) less.sheets.splice(index, 1);
    var node = document.getElementById(getId(link));
    if (node) node.parentNode.removeChild(node);
    return link;
  }

  less.attach = attach;
  less.detach = detach;
}());