Add the shared preference keys and appearance values

src/Id/Enums/Appearance.php

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+<?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];
+    }
+}

src/Id/Enums/SharedPreference.php

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+<?php
+
+namespace Anikeen\Id\Enums;
+
+/**
+ * The keys of the `global` preferences namespace, shared by all apps. Anikeen ID accepts only
+ * these keys there, each with its own values.
+ */
+class SharedPreference
+{
+    /** The namespace of the shared preferences. */
+    const NAMESPACE = 'global';
+
+    /** Light or dark mode: one of the Appearance constants. */
+    const APPEARANCE = 'appearance';
+}

tests/Id/SharedPreferenceTest.php

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+<?php
+
+namespace Anikeen\Id\Tests;
+
+use Anikeen\Id\Enums\Appearance;
+use Anikeen\Id\Enums\SharedPreference;
+
+class SharedPreferenceTest extends TestCase
+{
+    public function test_the_shared_keys_and_values_match_anikeen_id(): void
+    {
+        $this->assertSame('global', SharedPreference::NAMESPACE);
+        $this->assertSame('appearance', SharedPreference::APPEARANCE);
+        $this->assertSame(['system', 'light', 'dark'], Appearance::all());
+    }
+}