Apple and Google use grey borders for their default (and preferred badges). 
https://cdn.satellite.earth/050a46d236c9e302126e2ccffe86ff00aab64576a8cc9526477321937e88743e.png

PNG for the button (old-style): 
https://cdn.satellite.earth/fdf0d4f36d9fee639ddfebac289a4f18efe3ceabffc89843caeb8f43ebe80cbf.png

PNG for the button (new Apple style): 
https://cdn.satellite.earth/84dde28e6b2f727ddbb85fdf31ce1fc6c2731edb571444e8fbe0cd0b4ebf2ee8.png