fonts - Is there a way to customize the CSS of Firefox Reader View?

I love Firefox Reader View. But not its fonts. More specifically, these are not the best choices:

body.serif, body.serif .remove-button {
    font-family: Georgia, "Times New Roman", serif;
}

body.sans-serif, body.sans-serif .remove-button {
    font-family: Helvetica, Arial, sans-serif;
}

I'd rather have this:

body.serif, body.serif .remove-button {
    font-family: "Cambria", serif;
}

body.sans-serif, body.sans-serif .remove-button {
    font-family: "Open Sans", "Segoe UI", "Tahoma", "Calibri", sans-serif;
}

Now, I can apply these changes manually every time I open the Reader View via the developers tools. But is there a way to automate this?

I tried Stylish, but either I could code it correctly or Stylish wasn't up to the task.

Answer

In case you haven't gotten around to hunting this down yet, and/or for posterity, as of FF v60 the solution is to:

• locate your profile folder (listed in about:support)


• in there, create a folder called "chrome" if it doesn't already exist


• in that folder, create a file called "userContent.css"

Then you'll need the correct CSS at-rule and you're good to go:

@-moz-document url-prefix("about:reader") {
    body.serif {
        font-family: 'Besley*' !important;
    }
}

N.B. you can also style the sidebar and its buttons, helpful to reduce contrast for dark themes:

.toolbar {
    background-color: #333 !important;
    border-right: 1px solid #444 !important;
}
.button {
    color: #ccc !important;
    background-color: #444 !important;
}