That repo looks more like a sounding board not actual working code (maybe I'm reading that wrong though).
The MODx manager has some
serious problems in this regard: for all the flexibility MODx offers on the front-end, the back-end manager is quite fixed in its ways. Even though you can use your own theme directory, it doesn't do you much good because 90%+ of the manager HTML is generated by Ext JS so the templates are little more than empty divs.
You might have better luck with
http://rtfm.modx.com/extras/revo/handyman -- it's not a manager theme so much as it is an alternate manager.