"We couldn’t find a single package of Lego blocks that didn’t have a theme"

I don't think they looked hard enough:

http://www.amazon.com/LEGO-Ultimate-Building-Set-Pieces/dp/B...

http://www.amazon.com/LEGO-Bricks-More-Builders-Tomorrow/dp/...

Those are cherrypicked examples. 15 years ago they still had the custom horse, dragon, stone wall, etc pieces and today they still have sets of just plain pieces.

Longer ago than that... More than 30 years ago there were themed sets with somewhat custom pieces:

http://letsbuilditagain.com/instructions/6630/ http://letsbuilditagain.com/instructions/6880/

(To pick 2 completely random examples)

Back in '82 there weren't as many different themes (afaik there were maybe 4: space, town, castle and trains), but the concept definitely existed.

Some might even argue about the gender-neutrality of those themes too: the majority of Lego I owned around that time was the space-themed stuff, and my sister had some of the town sets. In fact, as long ago as 1979 Lego brought out a range called Scala [1] that was targeted almost exclusively at girls (my wife has really fond memories of those sets!)

[1] http://lego.wikia.com/wiki/Scala

that's not a comparison it's at best an anecdote.