This bug applies to when you have the dark theme set in your Display Settings. By clicking on the link of an image that has been embedded in a post (after clicking on it to expand it), and then going back to the previous page by opening a new tab (middle mouse click on Back button), the page you return to will have the default background color (white) instead of the dark theme. I am using FireFox 21.0, and exists in FireFox 22.0