Tweak fallback/non-Google Font appearance

Rather than just falling back on a font stack, let’s tweak the styles of
`system-ui` so that they look decent in their own right.

Author: csswizardry

demo/index.html

@@ -60,6 +60,12 @@
       font-weight: 500;
     }
 
+    .has-delivery-mode-lite h1,
+    .has-delivery-mode-lite h2 {
+      font-family: system-ui, sans-serif;
+      font-weight: 700;
+    }
+
     h1 {
       font-size: 2.25rem;
       line-height: 1.333333333;