Propose variations : serif, sans-serif, monospace ?

This approach is splendid.

What about some variations; as in many styles / frameworks / design systems, a "theme" is composed of several fonts, including some serif, some sans-serif and some monospaced.

Furthermore; I saw you're using the exact same technique on your homepage index.html to define a "system monospace font" :

```
@font-face {
font-family: monospace;
font-style: normal;
font-weight: 400;
src: local("Menlo-Regular"), local("Monaco"), local("Consolas"), local("DejaVu Sans Mono"), local("DejaVu Sans Mono");
}

@font-face {
font-family: monospace;
font-style: italic;
[...]
```

What do you think ?

`system-serif`
`system-sans-serif`
`system-mono`

I can PR this, though I don't have time to redo the research to find the system's serif and mono fonts. Can you help me ?