For Under Armour, the story was always the same: Form follows function. The Baltimore sports apparel company's gear was designed to keep you sweat-free, while its tight fit showed off the muscular physiques of those lucky enough to have them. The brand was never known for being fashion-forward. But that was then. Now the billion-dollar firm, which as recently as three years ago limited its clothing offerings to shirts, shorts and other athletic wear in only the most basic colors, is looking to up its game — and expand its business.