body {
background: rgba(0, 0, 0, 0.8);
font-family: sans-s
Take a look at this fiddle. http://jsfiddle.net/nUk77/ Or even closer to your original code: http://jsfiddle.net/rSbzT/
Instead of absolute positioning the register div I positioned its container. The absolute positioning makes it break from the html flow so it doesn't interfere with the margin-right:auto;