Converting nginx reverse proxy (with CSS filter) to IIS10 URL Rewrite

I’m currently using nginx (for Windows) to handle SSL as well as handling my reverse proxies. Below, is an example reverse proxy code for nginx (which also includes a CSS mod):

        location /files {
            proxy_pass http://localhost:999/files;
            proxy_set_header Accept-Encoding "";
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection $http_connection;
            sub_filter
            '</head>'
            '<link rel="stylesheet" type="text/css" href="https://github.io/test/CSS/themes/dark.css">
            
            <style>
            .CalendarEvent/downloaded/2vSrJ {
                background: rgba(39, 194, 76, 0.7) !important;
                border-left-color: rgba(39, 194, 76, 0.0) !important;
            }
            </style>

            </head>';
            sub_filter_once on;
        }

Can someone please show me how to setup the same reverse proxy using IIS URL Rewrite? I’m particularly curious about how to handle the CSS mods above. I learn best from real working examples. If someone can post a screenshot, that would be ideal.

PS: Are there any performance benefits/disadvantages switching to IIS URL Rewrite over nginx?