Add the shared preference keys and appearance values

src/Id/Enums/Appearance.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Anikeen\Id\Enums;
+
+/**
+ * The values of the shared preference SharedPreference::APPEARANCE.
+ */
+class Appearance
+{
+    /** Follow the operating system. */
+    const SYSTEM = 'system';
+    const LIGHT = 'light';
+    const DARK = 'dark';
+
+    /**
+     * @return list<string>
+     */
+    public static function all(): array
+    {
+        return [self::SYSTEM, self::LIGHT, self::DARK];
+    }
+}