Default internal font, CSS rules

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I think in most cases it is being defined simply as sans-serif, meaning it will depend on your operating system. I am not sure if that is true of all of the built in themes.

As far as I know, no one has documented all the rules. A bunch of us have shared our customizations here: Share your CSS

I've considered working on documenting the CSS better, but there is work in progress to implement a new base theme, so I want to wait to see what comes from that.

To change your base font for the rendered note, add the following to userstyle.css

body{
    font-family: "your-font-name" !important;
}

For the rest of the app you can add the following to userchrome.css (this assumes you use the ace editor):

* {
 font-family: "your-font-name" !important;
}
.ace_editor * {
    font-family: "monospace-font" !important;
}

Both of these files are accessible from Settings > Appearance > Advanced.

To find other available CSS styles: Dev tools instructions