Adidas had slipped far behind Nike and was even passed by Under Armour, a much younger company, in North American sales, but the German brand has staged a recent comeback by partnering with pop culture titans such as Kanye West, as well as by adding hip new styles to its retro-chic classics. Kaepernick, whose post-NFL accolades include being named GQ magazine's "Citizen of the Year" in November, could appeal to some of the same demographic, at least in theory, without ever having to step behind center again.