Save webpage look modified by userContent.css with Singlefile?

[FFnewbie]

I see this userContent.css mod on substack working perfectly:

@-moz-document domain(substack.com){

• { font-family : Segoe UI Semibold Regular !important; font-style: normal !important;
  }
  }

I want to archive my favorite substack page WITH the above LOOK, but SingleFile creates the original - unmodified CSS website look, as stored on the substack server.
But I want to save my modified CSS look with SingleFile!

Is that possible? Like somehow inject/load a domain related CSS-mod into SingleFile, so the altered webpage-look is saved exactly as I see it on the screen? Instead of the original webpage look/design.

[FFnewbie]

SOLVED:
Looked up on the net, how to insert LINK TAG into HTML.
https://www.freecodecamp.org/news/how-to-link-css-to-html/

1. Put above code - in 1st post - into a text file, named it userContent.css and saved it right next to substack webpage saved by SingleFile in a custom folder.
2. Opened the substack webpage saved by SingleFile in text editor:

<meta name=norton-safeweb-site-verification 
content=24usqpep0ejc5w6hod3dulxwciwp0djs6c6ufp96av...etc
> <style>:root{--sf-img-14: 
url("data:image/png;base64,iVBORw0KGgoAAAANSUhEU....etc

3. Inserted link tag, so it now looks like this:

<meta name=norton-safeweb-site-verification 
content=24usqpep0ejc5w6hod3dulxwciwp0djs6c6ufp96av...etc
<link rel="stylesheet" type="text/css" href="userContent.css" />
<style>:root{--sf-img-14: 
url("data:image/png;base64,iVBORw0KGgoAAAANSUhEU....etc

5. In my text editor I saved the the substack webpage saved by SingleFile and loaded it again in my browser.

6. Now it looks exactly how I want and I can read it without eyestrain!!

ISSUE CLOSED