How to convert an nginx reverse proxy with respective "sub_filter" to IIS?

I'm currently using the below nginx reverse proxy with sub_filter.  This allows me to quickly create a reverse proxy along with a respective CSS filter in realtime.

I can create the equivalent reverse proxy in an IIS URL Rewrite rule relatively easily.  However, I have no idea how to also add the respective nginx sub_filter code (shown below) to the reverse proxy I added in IIS.  I'm guessing this can be done with URL rewrite outbound rule.

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;

}

Maybe, I need to add a request filtering rule in IIS? Whatever it is, I have no clue how to do this under IIS. I learn from working examples.  If someone can show me what to add specifically to IIS, I'd really appreciate it.  Surely there has to be a way to do this in IIS.

PS: I asked on StackOverflow a similar question, in case you would also like to get credit for a StackOverflow answer:

Converting nginx reverse proxy (with sub_filter) to IIS10 URL Rewrite / Request Filtering

Thanks in advance to anyone who can figure out how to do this.

Below, is the current reverse proxy rewrite code I have currently in my web.config:

<rewrite><rules><rule name="Redirect to HTTPS" enabled="false" stopProcessing="true"><match url="(.*)" /><conditions><add input="{HTTPS}" pattern="^OFF$" /><add input="{REMOTE_ADDR}" pattern="192.168.1.2" negate="true" /></conditions><action type="Redirect" url="https://{HTTP_HOST}/{R:1}" redirectType="Permanent" /></rule><rule name="ReverseProxy-FILES" enabled="false" stopProcessing="false"><match url="files\/?(.*)" /><action type="Rewrite" url="http://localhost:999/files/{R:1}" /></rule></rules></rewrite>