Add standard font "system-ui" for body to style.css

[1jj]

https://hstspreload.org/static/css/style.css has this font stack:
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Helvetica, Arial, sans-serif;

I propose to change it to this:
font-family: system-ui, "-apple-system", "Segoe UI", Roboto, "Helvetica Neue", Helvetica, Arial, sans-serif;

Because system-ui is the standard version of prefixed BlinkMacSystemFont and -apple-system. It is implemented since Chrome 56:
https://www.chromestatus.com/feature/5640395337760768
and also in Safari 11:
https://caniuse.com/#feat=font-family-system-ui

As good web citizens you should use the standard way and ideally drop the prefixed fonts (may be too early for Safari to drop the apple prefix?)

[nharper]

Feel free to send a pull request with that change.

[XhmikosR]

Just keep in mind that this change was aiming at the best compatibility (I was the one who made the change to the native font stack). I made #177 where I added Noto Sans, so feel free to make a PR to propose your changes.