BY MARGIT DETWEILER If you had told 20-something me that one day I would be writing about my fresh pair of Birkenstocks, I would have spat out my Fresca. Back in the ‘90s I was committed to combat boots, black platform slides, anything black and chunky. My Grateful Dead-following best friend/ roomie had a pair of fuzzy Birkenstocks which I nastily dubbed her “bear paws” or “birth-control shoes.” She’d protest, “But they’re comfeeeee.”