detect_night_mode.user

// ==UserScript==
// @name         Twitter - CSS Class for Night Mode
// @namespace    http://xt-8147.blogspot.com/
// @version      1.0
// @description  Detects if you're using Night Mode on Twitter and sticks a CSS class on the body tag.
// @author       XT-8147
// @include      https://twitter.com/*
// @grant        none
// @run-at       document-start
// ==/UserScript==

/* Version History
 * Version  Notes
 *   0.1    * Original version.
 *   0.2    * Properly adds and removes the CSS class as necessary after page load, since the user is free to toggle it at will after page load.
 *   0.3    * The head actually mutates twice every time Night Mode is toggled, the toggle doesn't complete until the second mutation.  Now detecting this.
 *   1.0    * Code declared release-worthy.
 */

function maintainCSSClass() {
 var isNightMode = ( document.querySelector( 'link[rel="stylesheet"][href*="nightmode"]' ) !== null );
 var pushingState = document.body.classList.contains( 'pushing-state' );
 if( !pushingState ) {
  if( isNightMode ) {
   document.body.classList.add( 'night-mode' );
  }
  else {
   document.body.classList.remove( 'night-mode' );
  }
 }
}

var o = new MutationObserver( maintainCSSClass );
o.observe( document.head, { characterData: false, attributes: false, childList: true, subtree: false } );