Copy paste this in your browsers' developer tools console while editing a tumblr theme in the built in tumblr html editor and esc+r will update the preview

update-preview-tumblr-shortcut.js

(function(){
  /**
   * inserts a div with text into <div id="log"></div> or whichever element you pass to second param.
   * @param {*} - thing - text to log.
   * @param {Object} - optional - DOMElement to append to.
   */ 
  function log(thing, _el){
    var el = _el || document.getElementById('log')
    ,   div = document.createElement('div');
  
    div.innerText = thing;
    el.appendChild(div);
  }
  
  /**
   * Change the contents of this function to do whatever it is you want it to do on your key command press.
   */
  function doThing(){
    log(registrar.vals());
    // if you're working within the tumblr editor and want esc + r (on a mac) to trigger the Update Preview action:
    jQuery('[data-action=update_preview]').not('.disabled').click();

  }
  
  /**
   * Object that keeps track of key presses.
   * @params {Number} - <a href="http://css-tricks.com/snippets/javascript/javascript-keycodes/" target="_blank">keycode</a>. Default key command is ```esc + r```.
   * @constructor
   */
  function Registrar(_keyA, _keyB){
    var that = this;
    this.keyA = _keyA || 27;
    this.keyB = _keyB || 82;
    this.keyAPressed = false;
    this.keyBPressed = false;
    
    this.keys = (function(){
      return Object.keys(that);
    })();
  }
  
  /**
   * reset registered keys to false
   * @return {Object}
   */
  Registrar.prototype.clear = function(){
    this.keyAPressed = false;
    this.keyBPressed = false;
    return this;
  }
    
  /**
   * Helper function to get current values of keys. Not essential, but useful for debugging.
   * @return {String}
   */
  Registrar.prototype.vals = function(){
    var str = '';
    for(var i = 0; i < this.keys.length; i++){
      if(typeof this[this.keys[i]] === 'boolean'){
        str += " " + this[this.keys[i]];
      }
    }
    return str;
  };
  



  (function initialize(){
    var registrar = new Registrar();
    document.addEventListener('keyup', function(e){
      var key = e.which;
      
      if( key === registrar.keyA ){
        registrar.keyAPressed = true;
      } else if( key === registrar.keyB){
        registrar.keyBPressed = true;
      } else {
        registrar.clear();
      }
    
      if(registrar.keyAPressed === true && registrar.keyBPressed === true){
        doThing();
        registrar.clear();
      } 
    })
  })();
})();