I created a simple Stylish

  @namespace url(http://www.w3.org/1999/xhtml);

  @-moz-document domain("www.youtransfer.io") {
  body{
    color:#000000;
    font-size: 18px;
  }

  }

Not San Francisco not San Francisco not San Francisco ;)

:D

How about Proxima Nova? (http://www.marksimonson.com/fonts/view/proxima-nova)

Simply "Sans" will work just fine.