switch.layer.borderWidth = 1.0;
switch.layer.cornerRadius = 16.0;
switch.layer.borderColor = [UIColor whiteColor].CGColor;
[self.mySwitch setThumbTintColor:[UIColor blueColor]];
[self.mySwitch setOnTintColor:[UIColor redColor]];

ie. Use setOnTintColor for the background/border color.
// s is a UISwitch
s.layer.borderWidth = 1
s.layer.borderColor = UIColor.whiteColor().CGColor
s.layer.cornerRadius = 16 // you must import QuartzCore to do this.
switch.tintColor = [UIColor redColor]; // the "off" color
switch.onTintColor = [UIColor greenColor]; // the "on" color