Auto switch style in 404 badge

libs/serve-404.ts

@@ -1,3 +1,4 @@
+import url from 'url'
 import serveBadge from './serve-badge'
 
 export default function (req, res) {
@@ -7,5 +8,14 @@ export default function (req, res) {
     color: 'orange'
   }
 
-  serveBadge(req, res, { code: 404, params })
+  const { query } = url.parse(req.url, true)
+
+  if (query.style === undefined) {
+    const host = req.headers['x-forwarded-host'] || req.headers.host
+    if (host.startsWith('flat')) {
+      query.style = 'flat'
+    }
+  }
+
+  serveBadge(req, res, { code: 404, params, query })
 }