给 html 元素添加下面的样式实现哀悼模式（黑白模式）
.sad-theme {
-webkit-filter: grayscale(100%);
-moz-filter: grayscale(100%);
-ms-filter: grayscale(100%);
-o-filter: grayscale(100%);
filter: grayscale(100%);
-webkit-filter: grey;
filter: gray;
filter: progid:DXImageTransform.Microsoft.BasicImage(grayscale=1);
}
