gistfile1.txt

$(function(){

    var $window = $(window);
    var $header = $('.site-header');
    var $windowHeight = $window.height();
    var $headerHeight = $windowHeight - 20;

    $header.css({'height': $headerHeight +'px'});
    $window.resize(function(){
        $header.css({'height': $headerHeight +'px'});
    });
});