This user script makes all Stack Exchange Q&A sites use the generic beta site theme

se_beta_everywhere.user.js

// ==UserScript==
// @name        Stack Exchange beta theme everywhere
// @namespace   http://vyznev.net/
// @description Makes all Stack Exchange sites use the generic beta site theme
// @author      Ilmari Karonen
// @version     0.5.2
// @license     Public domain
// @homepageURL http://meta.stackexchange.com/a/275804
// @match       *://*.stackexchange.com/*
// @match       *://*.stackoverflow.com/*
// @match       *://*.superuser.com/*
// @match       *://*.serverfault.com/*
// @match       *://*.stackapps.com/*
// @match       *://*.mathoverflow.net/*
// @match       *://*.askubuntu.com/*
// @exclude     *://stackexchange.com/*
// @exclude     *://chat.*
// @exclude     *://blog.*
// @exclude     *://area51.*
// @exclude     *://careers.*
// @exclude     *://openid.*
// @exclude     *://data.*
// @run-at      document-start
// @grant       none
// ==/UserScript==

var skin = 'beta';
var meta = (/(^|\.)(meta|discuss)\./.test(location.hostname) ? 'meta' : '');

function observeElement ( element, callback ) {
  if ( callback() ) return;
  var observer = new MutationObserver( function () {
    if ( callback() ) observer.disconnect();
  } );
  observer.observe( element, { childList: true, subtree: false } );
}

observeElement( document.documentElement, function () {
  var head = document.head;
  if (head) observeElement( head, function () {
    var link = head.querySelector('link[rel=stylesheet][href*="/all.css"]');
    if (link) link.href = '//cdn.sstatic.net/Sites/' + skin + meta + '/all.css';
    return link;
  } );
  return head;
} );

var extraStyle = document.createElement('style');
extraStyle.textContent = '#custom-content { display: none }';
(document.head || document.documentElement).appendChild(extraStyle);