From the Official Website:
Welcome to Yesterday's Tomorrow, a retro sci-fi roleplaying game. I designed this game for Ro Watts because she asked me for a steam punk game. I told her, "You don't want a steam punk game; you want a game about how the past thought the future might be." I said this because she loves movies like Sky Captain and the World of Tomorrow and games like Bioshock. Both that movie and that game aren't steam punk, but they are about a speculative future. A future from the past's point of view.
And so, this game is about just that: how the past thought the future would look like. But it's also about a little more.