why A Virus Named Tom has a different background on SG

Several games have different backgrounds here, typically from massive dev giveaways. For example, both Faerie Solitaire and Dwarfs have special backgrounds as well.

Because, as the title says, the game is a virus (namely Tom.Worm.32.GBN). It targets webhostings, one of the notable effects is replacing header picture with its own (which is, in fact, infected .rarjpeg).

tldr: interwebz are dark and full of viruses.