Result

jQuery

$(document).ready(function(){
   //高さを取得
   var docHeight = $(document).height();
   //全体にオーバーレイ用のdivを加える
   $("body").append("<div id='overlay'></div>");

   $("#overlay")
       //オーバーレイ用に追加したdivにcssを加える。普通はクリックイベントと併用します
       //最初からcssに記載しといた方がスッキリしますね。
      .height(docHeight)
      .css({
         'opacity' : 0.4,
         'position': 'absolute',
         'top': 0,
         'left': 0,
         'background-color': 'black',
         'width': '100%',
         'z-index': 5000
      });
});

via

Append Site Overlay DIV | CSS-Tricks

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