var $bg = $('<div class="dialog-bg"></div>');
var $dialog = $('<div class="dialog"></div>');
var $title = $('<div class="title"></div>');
var $span = $('<span></span>');

this.body.append($bg);

$span.html(this.title).appendTo($title);
this.child.wrap($dialog);
this.child.before($title);
this.child.addClass('content');

$bg.css('margin-left', '-300px'); //To działa
$dialog.css('margin-left', '-300px'); //To nie działa

.dialog-bg{
position: absolute;
top: 0px;
left: 0px;
width: 100%;
height: 100%;
background: rgba(0,0,0,0.8);
z-index: 100;
}
.dialog{
position: absolute;
top: 50px;
left: 50%;
z-index: 1000;
background: white;
border-radius: 3px;
width: 500px;
}