No not use per-browser settings for the theme to avoid flashing pages on navigation.

Signed-off-by: DL6ER <dl6er@dl6er.de>

scripts/pi-hole/php/theme.php

@@ -0,0 +1,47 @@
+<?php
+/* Pi-hole: A black hole for Internet advertisements
+*  (c) 2020 Pi-hole, LLC (https://pi-hole.net)
+*  Network-wide ad blocking via your own hardware.
+*
+*  This file is copyright under the latest version of the EUPL.
+*  Please see LICENSE file for your rights under this license. */
+
+// Array of available themes and their description
+$available_themes = [];
+/* Array key = name used internally, not shown to the user
+*  Array[0] = Description
+*  Array[1] = Is this a dark mode theme? (Sets background to black during page reloading to avoid white "flashing")
+*  Array[2] = Style sheet name
+*/
+$available_themes["default-light"] = ["Pi-hole default theme (light, default)", false, "default-light"];
+$available_themes["default-dark"] = ["Pi-hole midnight theme (dark)", true, "default-dark"];
+
+$webtheme = "";
+// Try to load theme settings from setupVars.conf
+if(isset($setupVars['WEBTHEME']))
+    $webtheme = $setupVars['WEBTHEME'];
+
+// May be overwritten by settings tab
+if(isset($_POST["field"]) &&
+   $_POST["field"] === "webUI" &&
+   isset($_POST["webtheme"])) {
+    $webtheme = $_POST["webtheme"];
+}
+
+if(!array_key_exists($webtheme,$available_themes)) {
+        // Fallback to default (light) theme is property is not set
+        // or requested theme is not among the available
+        $webtheme = "default-light";
+}
+
+$darkmode = $available_themes[$webtheme][1];
+$theme = $available_themes[$webtheme][2];
+
+function theme_selection() {
+    global $available_themes, $webtheme;
+    foreach ($available_themes as $key => $value) {
+        ?><div><input type="radio" name="webtheme" value="<?php echo $key; ?>" id="webtheme_<?php echo $key; ?>" <?php if ($key === $webtheme){ ?>checked<?php } ?>>
+        <label for="webtheme_<?php echo $key; ?>"><strong><?php echo $value[0]; ?></strong></label></div><?php
+    }
+}
+?>