Project Two: Website page border

Originally when I designed my website I wanted to have a 10mm white border around my whole website page. I didn't want it change either when the user zoomed in or out. I managed to get the border looking right on my computer but realised it wont be the same on computers with bigger screens. At the moment I am trying to figure out how I can do this. I have applied a border using the border code but I want to figure out how I can apply it so that when you zoom in and out it adjusts it self accordingly and still has that same border. Apart from that I think my website is finished I just need some people to test it and see if they pick up any mistakes or recommend changing / adding something.