Issue #3388329 by jurgenhaas: Update to 5.0.7 breaks sass because of missing $font-size-xs variable

scss/typography.scss

@@ -20,6 +20,7 @@ $font-family-sans-serif:      -apple-system, BlinkMacSystemFont, "Segoe UI", Rob
 $font-family-monospace:       SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
 $font-size-root:              null;
 $font-size-base:              1rem; // Assumes the browser default, typically `16px`
+$font-size-xs:                $font-size-base * .75;
 $font-size-sm:                $font-size-base * .875;
 $font-size-lg:                $font-size-base * 1.25;