Why is my background for Wikipedia the website light blue?

I don't know how I did it but I just opened a Wikipedia page and suddenly the background of the website (page) went light blue! No I didn't copy-and-paste anything as I was just viewing the page. Any help?
Update: Once again, I did not copy or paste ANYTHING at all. I just viewed a Wikipedia page and it suddenly changed the background of the page.
3 answers 3