nginx.conf

# 1. Make sure you have nginx sub module compiled in
# nginx -V  2>&1 | grep --color=always '\-\-with\-http_sub_module'

# 2. add two directives below at HTTP level

# nginx.conf
http {
        # ......

        sub_filter  '</head>' '<style type="text/css">html{ filter:progid:DXImageTransform.Microsoft.BasicImage(grayscale=1); -webkit-filter: grayscale(100%); } img { _filter:progid:DXImageTransform.Microsoft.BasicImage(grayscale=0); -webkit-filter: grayscale(100%); } </style>';
        sub_filter_once on;

        # ......
}

# 3. nginx -t && /etc/init.d/nginx reload

Notes:

1. Does not work on Firefox