响应Div圈内另一个响应div圈并集中？

It's because in fact I need a 5px or 10px border. Researching I figured out that borders cannot use percents - So, the alternative is to create a circle with a circle of 94% inside. but I cannot centralize it :( I can put this on center using side margins to auto, but I'm still can't get it centralised vertically. Any clue about the best way to put one circle inside another to simulate a border ?It's because in fact I need a 5px or 10px borde