Use backwards-compatible font stack for base theme

Author: lachlanjc

packages/preset-base/src/index.js

@@ -8,7 +8,7 @@ const heading = {
 export const base = {
   space: [0, 4, 8, 16, 32, 64, 128, 256, 512],
   fonts: {
-    body: 'system-ui, sans-serif',
+    body: 'system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", sans-serif',
     heading: 'inherit',
     monospace: 'Menlo, monospace',
   },