Relative size header using CSS

I have designed the header with the following CSS:

.Navigation { height: 84px; background: url(../images/Navigation.png) repeat-x; border-bottom: 1px solid #919191; }

#NavigationInside { padding-left: 15px; width: 945px; margin: 0 auto; }

This 开发者_如何学Cis fixed height and width header whereas I want it relative to the screen size. Both end of the header should touch the left and right corner of the screen.

Please help me out to make changes in the CSS.

.Navigation { 
  width:100%; 
  margin:0; 
  padding:0; 
  ...