Are contemporary chess players now stronger than players in the 90's or 80's? For example, take a Class A (a player with a USCF rating of between 1800 and 1999 inclusive) today and Class A from the 90's. Would the contemporary Class A be stronger because of the updates of opening theory or contemporary changes in chess play?
No. The vast majority of club level players still play like the vast majority of club level players always did. Changes in knowledge about chess mostly apply to high levels of play; the kind of mistakes class A players make were already known to be mistakes for ages, and opening theory doesn't help much if your opponent deviates from theory at move 5 anyway.

As evidence of this, see the paper "Intrinsic Chess Ratings" by Regan and Haworth, PDF. They analyzed the games of players of various rating ranges from various time periods with an engine, and found no significant difference in the mistake rate between games from the 70s and from the 2000s.