As explained by Gabriel Auditore already, this is likely caused by a background invasive advert. Registering on the site will relinquish these adverts, however should you not want an account, browsers such as Chrome and Firefox offer free add block add-ons which will hide these adverts in future.
I'm also noticing the same thing, and I have ads turned off in my preferences as well. A look at the source reveals that the problem is not due to ads at all, but because the background color for the terminal blocks is #DDD, a very light grey. I'm going to change it to a much darker grey, though still lighter than the main background color.