I'm using the 29.0.1547.41 beta version of Chrome OS on a Samsung Chromebook. Clicking the toggle to switch the site to the dark theme does nothing. If I click the light theme toggle button I can see site elements kind of twitch like they do on my desktop when I switch themes, so that button is apparently working fine. I've restarted my Chromebook just to see if the browser was acting up but it didn't help. I have no idea if this is a site quirk or a Chrome OS issue (considering it is a beta version, I guess it's likely on my end).