change remark42 theme based on browser theme

_includes/comments/remark42.html

@@ -21,6 +21,12 @@
             (d.head || d.body).appendChild(s);
         }
     })(remark_config.components || ['embed']);
+
+    window.onload = function () {
+        if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
+            window.REMARK42.changeTheme('dark');
+        }
+    }
 </script>
 
 <div id="remark42"></div>