fix variable-font-size mixin

src/client/styles/scss/_mixins.scss

@@ -10,7 +10,7 @@
   @include media-breakpoint-only(lg) {
     font-size: #{$basesize * 0.9};
   }
-  @include media-breakpoint-only(xl) {
+  @include media-breakpoint-up(xl) {
     font-size: $basesize;
   }
 }