set color-scheme in root

Author: mbostock

test/plots/index.html

@@ -3,6 +3,10 @@
   <meta charset="utf-8" />
 </head>
 <style>
+  :root {
+    color-scheme: light dark;
+  }
+
   body {
     max-width: 960px;
     margin: 1em auto;