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).
Yep, it's been messed up since launch. Not sure why it hasn't been fixed.
Another workaround is hovering over the black button, then refreshing your browser, then clicking the button after the browser is refreshed without moving your pointer.
Bugs are odd things.