Center modal dialogue box and decrease width

Hi,

If you look at this page http://goo.gl/MIobJD and click on the download button a modal dialogue appears.

I would like to decrease the width of the modal dialogue t about 306px and have it centred and maintain the centred potion on any screen size, so it is always in the middle of the screen.

I have attempted a few different css code styles but cannot get it to always be centred if screen size changes.

If you could assist with css styling I would appreciate it.

Thanks