Even if you don't have jQuery available, you can use the add and removeClass() functions!

jqAddAndRemoveClass.js

function jQaddClass( elem, value ) {
  var newClass = value;

  if ( elem.nodeType === 1 ) {
    if ( !elem.className ) {
      elem.className = value;
    } else {
      var className = " " + elem.className + " ", setClass = elem.className;

      if ( className.indexOf( " " + newClass + " " ) < 0 ) {
        setClass += " " + newClass;
      }
      elem.className = setClass.replace(/^ */, "").replace(/ *$/, "");
    }
  }
}

function jQremoveClass( elem, value ) {
  var oldClass = value;
  var rclass = /[\n\t\r]/g; //from jQuery's attributes.js

  if ( elem.nodeType === 1 && elem.className ) {
    if ( value ) {
      var className = (" " + elem.className + " ").replace(rclass, " ");
      className = className.replace(" " + oldClass + " ", " ");
      elem.className = className.replace(/^ */, "").replace(/ *$/, "");

    } else {
      elem.className = "";
    }
  }
}