I think that this would be a security nightmare and would require a massive redesign for session cookies.
If anything they should be trying to lock cookies and sites assorted data to the specific device used to log in.
At the moment it is very easy for a criminal (once they have gained access) to nab your browser's entire profile and load it up on their computer giving them access to everything logged in on that profile.
What you are suggesting is something that would make the criminals jobs easier .
Twitch already has a dark mode available in the settings.