Today is the day [2017-06-06], when Twitch.tv disabled the way I was switching UI. If I figure a new way, I will update this extension. For now it only changes colors of UI.
Extension recolors new Twitch.tv top navbar and left sidebar to dark colors. It also adds menu link to switch between old and new Twitch.tv website as long as Twitch.tv supports it.
For more info visit: https://twitch.findl.sk/
Extension permissions explained:
*://*.twitch.tv/* - add custom css and js to website
(c) Oliver Findl 2017 https://www.findl.sk/