css:

body: margin-top:XXpx;

.page_header_container: position:fixed;

js:
var $window = $(window),
$body = $(‘body‘),
$bgMask = $(‘#page_header_container .bgMask‘);
$window.on(‘scroll‘, function() {
if (parseInt($window.scrollTop()) > 0) {
$bgMask.css(‘opacity‘, ‘.7‘);
} else {
$bgMask.css(‘opacity‘, ‘1‘);
}
});

頁面滾動後 頂部的透明度變化