add darkmode cookie

static/js/auto_darkmode.js

@@ -1,4 +1,59 @@
-let darkmode = window.matchMedia('(prefers-color-scheme: dark)').matches;
-if (darkmode) {
+function setCookie(cname, cvalue) {
+    document.cookie = cname + "=" + cvalue + ";path=/;";
+}
+
+function getCookie(cname) {
+    let name = cname + "=";
+    let decodedCookie = decodeURIComponent(document.cookie);
+    let ca = decodedCookie.split(';');
+    for(let i = 0; i <ca.length; i++) {
+      let c = ca[i];
+      while (c.charAt(0) == ' ') {
+        c = c.substring(1);
+      }
+      if (c.indexOf(name) == 0) {
+        return c.substring(name.length, c.length);
+      }
+    }
+    return "";
+}
+
+function getDarkmodeSetting() {
+    switch(getCookie("darkmode")) {
+        case "true":
+            return true;
+        case "false":
+            return false;
+        default:
+            return null;
+    }
+}
+
+function setDarkmodeSetting(value) {
+    if (value) {
+        setCookie("darkmode", "true");
+    } else {
+        setCookie("darkmode", "false");
+    }
+}
+
+function switchDarkmodeSetting() {
+    let darkmode = getDarkmodeSetting();
+    switch(darkmode) {
+        case true:
+            setDarkmodeSetting(false);
+            break;
+        case false:
+            setDarkmodeSetting(true);
+            break;
+        default:
+            setDarkmodeSetting(true);
+            break;
+    }     
+}
+
+let darkmode = getDarkmodeSetting();
+let darkmode_prefer = window.matchMedia('(prefers-color-scheme: dark)').matches;
+if ((darkmode_prefer && darkmode===null) || darkmode) {
     $(".darkmode-label").click();
 }