add prefers-color-scheme: dark to guestbook

style/guestbook.css

@@ -6,6 +6,20 @@
     --border: #DB7093;
 }
 
+@media (prefers-color-scheme: dark) {
+    :root {
+	--border: #474973;
+	--pagecolor: #474973;
+	--button-border-color: #474973;
+	--button-color: #474973;
+	--pagebg: #f1dac4;
+    }
+
+    input[type=submit] {
+	color: #f1dac4;
+    }
+}
+
 ul.pages {
     list-style:none;
     margin:auto;