The brown and tan background colours have always been the forum's background since it was first opened with the Lodestone during 1.0 - the original 1.0 Lodestone (still accessible here but no longer updated) had a very old-fashioned style to it's background so it's likely SE just went with the same colour for the forum as a part of that. I don't mind it to be honest - it keeps a connection to 1.0 alive even if most players don't notice it.