support night mode when using share by URL

js/utility.js

@@ -0,0 +1,39 @@
+Event.observe(window, "load", function() {
+    const UtilityJS = {
+        apply_night_mode: function (is_night, link) {
+            console.log("night mode changed to", is_night);
+
+            if (link) {
+                const css_override = is_night ? "themes/night.css" : "css/default.css";
+
+                link.setAttribute("href", css_override + "?" + Date.now());
+            }
+        },
+        setup_night_mode: function() {
+            const mql = window.matchMedia('(prefers-color-scheme: dark)');
+
+            const link = new Element("link", {
+                rel: "stylesheet",
+                id: "theme_auto_css"
+            });
+
+            link.onload = function() {
+                document.querySelector("body").removeClassName("css_loading");
+            };
+
+            try {
+                mql.addEventListener("change", () => {
+                    UtilityJS.apply_night_mode(mql.matches, link);
+                });
+            } catch (e) {
+                console.warn("exception while trying to set MQL event listener");
+            }
+
+            document.querySelector("head").appendChild(link);
+
+            UtilityJS.apply_night_mode(mql.matches, link);
+        }
+    };
+
+    UtilityJS.setup_night_mode();
+});