You could try creating a userstyle for about:reader with the Stylish add-on.

https://addons.mozilla.org/en-us/firefox/addon/stylish/

https://userstyles.org/

I'm actually not sure this will work. Reader View is so new and rarely used that nobody has tried this before (to my knowledge). Let me know if that works!

See also:

• chrome://global/skin/aboutReaderControls.css
• chrome://global/skin/aboutReaderContent.css

The main selector is #moz-reader-content

Mark Schmidt said

You could try creating a userstyle for about:reader with the Stylish add-on. https://addons.mozilla.org/en-us/firefox/addon/stylish/ https://userstyles.org/ I'm actually not sure this will work. Reader View is so new and rarely used that nobody has tried this before (to my knowledge). Let me know if that works!

Yes, this works. In Stylish, you can create a new style with something like this, according to taste: @namespace url(http://www.w3.org/1999/xhtml);

@-moz-document url-prefix("about:reader?") {

 #moz-reader-content {
   text-align: justify;
 }
 body.sans-serif{
   font-family: "Myriad Pro";
 }
 body.serif{
   font-family: "Adobe Garamond Pro";
 }
 body.light {
   color: #1a2030;
   background-color: #f9f6f0;
 }
 body.dark {
 
 }
 body.sepia {
 
 }

}

I have to say, defaulting to Arial doesn't make for a comfortable reading experience. This rule makes it a lot smoother.

(N.B: the fonts I chose above come with Adobe products, they're not standard. You'll have to pick fonts that are actually on your system.)