Add 'system-ui' to font stack.

docs/content/reboot.md

@@ -41,7 +41,9 @@ The default web fonts (Helvetica Neue, Helvetica, and Arial) have been dropped i
 $font-family-sans-serif:
   // Safari for OS X and iOS (San Francisco)
   -apple-system,
-  // Chrome for OS X (San Francisco)
+  // Chrome >= 56 for OS X (San Francisco), Windows, Linux and Android
+  system-ui,
+  // Chrome < 56 for OS X (San Francisco)
   BlinkMacSystemFont,
   // Windows
   "Segoe UI",

scss/_variables.scss

@@ -225,7 +225,7 @@ $grid-gutter-widths: (
 //
 // Font, line-height, and color for body text, headings, and more.
 
-$font-family-sans-serif: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif !default;
+$font-family-sans-serif: -apple-system, system-ui, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif !default;
 $font-family-serif:      Georgia, "Times New Roman", Times, serif !default;
 $font-family-monospace:  Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace !default;
 $font-family-base:       $font-family-sans-serif !default;