Styles: support user-specified base font size

[justjake]

I would like the styles to support user-specified base font size, and ideally opt-in to Apple's dynamic system font size. This is a nice and simple guide: https://dev.to/colingourlay/how-to-support-apple-s-dynamic-text-in-your-web-content-with-css-40c0

It's a 2-step process:

1. Add @supports (font: -apple-system-body) { html { font: -apple-system-body; } }. This will set the room em size based on the user's accessibility preference on Apple devices.
2. Specify the rest of relevant parts of your layout using rem units.

I may contribute this once issue #2 is resolved.