Sorry if it's always been this way - when did the background of the site turn to graph lines? Was it always like this and I didn't notice? Or did it change recently? It looks different but I feel like I could have just never noticed. I did just get a new laptop so maybe it shows up on this one.
This design element is present since the first time this site got its own design (so around end of 2010, details in the post linked by Martin). Depending on the screen it is not very visible. It may be that you just never noticed.