I just thought my browser was hacked:
But it appeared to be only working for me on a single site. I saw a screenshot from another user who experienced this on Super User, but when I open the homepage, I still see the familiar theme.
After a little bit of experimenting, the Time Travel mode seems to work only on question 'detail' pages. Other pages, such as question lists, the help center, review queues are unaffected. It doesn't matter if you open the question page from a question list (e.g. the home page) or navigate directly to a URL.