Add body classes for browser, OS and retina display support.

body_classes.js

$(function() {
    var browser, classList, os, platform, retina;
    retina = window.devicePixelRatio > 1 ? "retina" : "no-retina";
    platform = window.navigator.userAgent;
    if (/Chrome/.test(platform)) {
      browser = "chrome";
    } else if (/Firefox/.test(platform)) {
      browser = "firefox";
    } else {
      browser = "unknown-browser";
    }
    if (/Mac/.test(platform)) {
      os = "mac";
    } else if (/Windows/.test(plaform)) {
      os = "windows";
    } else {
      os = "unknown-os";
    }
    classList = os + " " + browser + " " + retina;
    $("body").addClass(classList);
    return false;
  });