diff --git a/assets/css/dark-theme.css b/assets/css/dark-theme.css
new file mode 100644
index 0000000..56832c5
--- /dev/null
+++ b/assets/css/dark-theme.css
@@ -0,0 +1,101 @@
+/**
+ * Dark palette for Twenty Twenty-Five default style variation.
+ * Overrides theme preset CSS variables when html.tt5-ldt-dark is set.
+ */
+
+html.tt5-ldt-dark {
+	color-scheme: dark;
+
+	--wp--preset--color--base: #111111;
+	--wp--preset--color--contrast: #f5f5f5;
+	--wp--preset--color--accent-1: #ffee58;
+	--wp--preset--color--accent-2: #d4a8d0;
+	--wp--preset--color--accent-3: #9b8fd4;
+	--wp--preset--color--accent-4: #a3a3a3;
+	--wp--preset--color--accent-5: #1c1c1a;
+	--wp--preset--color--accent-6: color-mix(in srgb, currentColor 25%, transparent);
+}
+
+html.tt5-ldt-dark body,
+html.tt5-ldt-dark .wp-site-blocks {
+	background-color: var(--wp--preset--color--base) !important;
+	color: var(--wp--preset--color--contrast) !important;
+}
+
+/* Blocks that use preset background/text colors */
+html.tt5-ldt-dark .has-base-background-color {
+	background-color: var(--wp--preset--color--base) !important;
+}
+
+html.tt5-ldt-dark .has-contrast-background-color {
+	background-color: var(--wp--preset--color--contrast) !important;
+}
+
+html.tt5-ldt-dark .has-base-color {
+	color: var(--wp--preset--color--base) !important;
+}
+
+html.tt5-ldt-dark .has-contrast-color {
+	color: var(--wp--preset--color--contrast) !important;
+}
+
+html.tt5-ldt-dark .has-accent-1-color {
+	color: var(--wp--preset--color--accent-1) !important;
+}
+
+html.tt5-ldt-dark .has-accent-2-color {
+	color: var(--wp--preset--color--accent-2) !important;
+}
+
+html.tt5-ldt-dark .has-accent-3-color {
+	color: var(--wp--preset--color--accent-3) !important;
+}
+
+html.tt5-ldt-dark .has-accent-4-color {
+	color: var(--wp--preset--color--accent-4) !important;
+}
+
+html.tt5-ldt-dark .has-accent-5-color {
+	color: var(--wp--preset--color--accent-5) !important;
+}
+
+html.tt5-ldt-dark .has-accent-1-background-color {
+	background-color: var(--wp--preset--color--accent-1) !important;
+}
+
+html.tt5-ldt-dark .has-accent-2-background-color {
+	background-color: var(--wp--preset--color--accent-2) !important;
+}
+
+html.tt5-ldt-dark .has-accent-3-background-color {
+	background-color: var(--wp--preset--color--accent-3) !important;
+}
+
+html.tt5-ldt-dark .has-accent-4-background-color {
+	background-color: var(--wp--preset--color--accent-4) !important;
+}
+
+html.tt5-ldt-dark .has-accent-5-background-color {
+	background-color: var(--wp--preset--color--accent-5) !important;
+}
+
+/* Form controls follow dark scheme */
+html.tt5-ldt-dark input:not([type="submit"]):not([type="button"]):not([type="reset"]),
+html.tt5-ldt-dark textarea,
+html.tt5-ldt-dark select {
+	background-color: #1a1a1a;
+	color: var(--wp--preset--color--contrast);
+	border-color: var(--wp--preset--color--accent-4);
+}
+
+html.tt5-ldt-dark ::selection {
+	background: var(--wp--preset--color--accent-3);
+	color: var(--wp--preset--color--contrast);
+}
+
+/* Smooth transition when toggling */
+html.tt5-ldt-animate,
+html.tt5-ldt-animate body,
+html.tt5-ldt-animate .wp-site-blocks {
+	transition: background-color 0.25s ease, color 0.25s ease;
+}
diff --git a/assets/css/toggle.css b/assets/css/toggle.css
new file mode 100644
index 0000000..b0d730b
--- /dev/null
+++ b/assets/css/toggle.css
@@ -0,0 +1,63 @@
+.tt5-ldt-toggle {
+	position: fixed;
+	z-index: 99999;
+	right: 1.25rem;
+	bottom: 1.25rem;
+	display: inline-flex;
+	align-items: center;
+	gap: 0.5rem;
+	padding: 0.65rem 1rem;
+	border: 1px solid color-mix(in srgb, currentColor 20%, transparent);
+	border-radius: 999px;
+	background: var(--wp--preset--color--base, #fff);
+	color: var(--wp--preset--color--contrast, #111);
+	box-shadow: 0 4px 20px rgba(0, 0, 0, 0.15);
+	font: 600 0.875rem/1 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
+	cursor: pointer;
+	transition: transform 0.15s ease, box-shadow 0.15s ease;
+}
+
+.tt5-ldt-toggle:hover {
+	transform: translateY(-2px);
+	box-shadow: 0 6px 24px rgba(0, 0, 0, 0.2);
+}
+
+.tt5-ldt-toggle:focus-visible {
+	outline: 2px solid var(--wp--preset--color--accent-3, #503aa8);
+	outline-offset: 3px;
+}
+
+.tt5-ldt-toggle__icon {
+	font-size: 1.1rem;
+	line-height: 1;
+}
+
+html:not(.tt5-ldt-dark) .tt5-ldt-toggle__icon--moon,
+html.tt5-ldt-dark .tt5-ldt-toggle__icon--sun {
+	display: none;
+}
+
+html.tt5-ldt-dark .tt5-ldt-toggle {
+	background: #1a1a1a;
+	color: #f5f5f5;
+	border-color: rgba(255, 255, 255, 0.15);
+}
+
+@media (max-width: 480px) {
+	.tt5-ldt-toggle__label {
+		position: absolute;
+		width: 1px;
+		height: 1px;
+		padding: 0;
+		margin: -1px;
+		overflow: hidden;
+		clip: rect(0, 0, 0, 0);
+		white-space: nowrap;
+		border: 0;
+	}
+
+	.tt5-ldt-toggle {
+		padding: 0.75rem;
+		border-radius: 50%;
+	}
+}
diff --git a/assets/js/toggle.js b/assets/js/toggle.js
new file mode 100644
index 0000000..d440a74
--- /dev/null
+++ b/assets/js/toggle.js
@@ -0,0 +1,125 @@
+(function () {
+  'use strict';
+
+  var config = window.tt5Ldt || {};
+  var btn = document.getElementById('tt5-ldt-toggle');
+  if (!btn) return;
+
+  var storageKey = config.storageKey || 'tt5_ldt_scheme';
+  var cookieName = config.cookieName || 'tt5_ldt_scheme';
+  var prefersDark = window.matchMedia('(prefers-color-scheme: dark)');
+
+  function getStoredScheme() {
+    try {
+      var stored = localStorage.getItem(storageKey);
+      if (stored === 'light' || stored === 'dark') {
+        return stored;
+      }
+    } catch (e) {}
+
+    var match = document.cookie.match(
+      new RegExp('(?:^|; )' + cookieName.replace(/[.*+?^${}()|[\]\\]/g, '\\$&') + '=([^;]*)')
+    );
+    if (match) {
+      var cookieVal = decodeURIComponent(match[1]);
+      if (cookieVal === 'light' || cookieVal === 'dark') {
+        return cookieVal;
+      }
+    }
+
+    return null;
+  }
+
+  function getScheme() {
+    var stored = getStoredScheme();
+    if (stored) {
+      return stored;
+    }
+    return document.documentElement.classList.contains('tt5-ldt-dark')
+      ? 'dark'
+      : 'light';
+  }
+
+  function setCookie(scheme) {
+    var path = config.cookiePath || '/';
+    var domain = config.cookieDomain ? '; domain=' + config.cookieDomain : '';
+    var maxAge = 60 * 60 * 24 * 365;
+    document.cookie =
+      cookieName +
+      '=' +
+      encodeURIComponent(scheme) +
+      ';path=' +
+      path +
+      domain +
+      ';max-age=' +
+      maxAge +
+      ';SameSite=Lax';
+  }
+
+  function applyScheme(scheme, animate) {
+    var root = document.documentElement;
+    var isDark = scheme === 'dark';
+
+    if (animate) {
+      root.classList.add('tt5-ldt-animate');
+      window.setTimeout(function () {
+        root.classList.remove('tt5-ldt-animate');
+      }, 300);
+    }
+
+    root.classList.toggle('tt5-ldt-dark', isDark);
+    root.setAttribute('data-tt5-scheme', scheme);
+
+    try {
+      localStorage.setItem(storageKey, scheme);
+    } catch (e) {}
+
+    setCookie(scheme);
+    updateButton(isDark);
+
+    if (config.isLoggedIn && config.ajaxUrl && config.nonce) {
+      var body = new FormData();
+      body.append('action', 'tt5_ldt_save_scheme');
+      body.append('nonce', config.nonce);
+      body.append('scheme', scheme);
+      fetch(config.ajaxUrl, { method: 'POST', body: body, credentials: 'same-origin' });
+    }
+  }
+
+  function updateButton(isDark) {
+    var i18n = config.i18n || {};
+    btn.setAttribute('aria-pressed', isDark ? 'true' : 'false');
+    btn.setAttribute(
+      'aria-label',
+      isDark ? i18n.light || 'Switch to light mode' : i18n.dark || 'Switch to dark mode'
+    );
+    btn.setAttribute(
+      'title',
+      isDark ? i18n.lightLabel || 'Light mode' : i18n.darkLabel || 'Dark mode'
+    );
+
+    var label = btn.querySelector('.tt5-ldt-toggle__label');
+    if (label) {
+      label.textContent = isDark ? i18n.lightLabel || 'Light' : i18n.darkLabel || 'Dark';
+    }
+  }
+
+  btn.addEventListener('click', function () {
+    var next = getScheme() === 'dark' ? 'light' : 'dark';
+    applyScheme(next, true);
+  });
+
+  // First visit: no saved preference — follow system setting until user toggles.
+  if (!getStoredScheme() && !document.documentElement.getAttribute('data-tt5-scheme')) {
+    applyScheme(prefersDark.matches ? 'dark' : 'light', false);
+  } else {
+    updateButton(document.documentElement.classList.contains('tt5-ldt-dark'));
+  }
+
+  prefersDark.addEventListener('change', function (event) {
+    if (getStoredScheme()) {
+      return;
+    }
+    applyScheme(event.matches ? 'dark' : 'light', true);
+  });
+})();
diff --git a/includes/class-tt5-ldt.php b/includes/class-tt5-ldt.php
new file mode 100644
index 0000000..912dc81
--- /dev/null
+++ b/includes/class-tt5-ldt.php
@@ -0,0 +1,253 @@
+<?php
+/**
+ * Main plugin bootstrap.
+ *
+ * @package TT5_Light_Dark_Toggle
+ */
+
+defined( 'ABSPATH' ) || exit;
+
+/**
+ * TT5 Light/Dark Toggle plugin.
+ */
+class TT5_LDT {
+
+	/**
+	 * Initialize hooks.
+	 */
+	public static function init() {
+		add_action( 'plugins_loaded', array( __CLASS__, 'bootstrap' ) );
+		add_action( 'admin_notices', array( __CLASS__, 'admin_theme_notice' ) );
+	}
+
+	/**
+	 * Load frontend assets when the active theme is Twenty Twenty-Five.
+	 */
+	public static function bootstrap() {
+		if ( ! self::is_target_theme() ) {
+			return;
+		}
+
+		add_action( 'wp_enqueue_scripts', array( __CLASS__, 'enqueue_assets' ) );
+		add_action( 'wp_footer', array( __CLASS__, 'render_toggle' ), 5 );
+		add_action( 'wp_head', array( __CLASS__, 'inline_preference_script' ), 0 );
+
+		add_action( 'wp_ajax_tt5_ldt_save_scheme', array( __CLASS__, 'ajax_save_scheme' ) );
+		add_action( 'wp_ajax_nopriv_tt5_ldt_save_scheme', array( __CLASS__, 'ajax_save_scheme' ) );
+
+		add_action( 'show_user_profile', array( __CLASS__, 'render_profile_field' ) );
+		add_action( 'edit_user_profile', array( __CLASS__, 'render_profile_field' ) );
+		add_action( 'personal_options_update', array( __CLASS__, 'save_profile_field' ) );
+		add_action( 'edit_user_profile_update', array( __CLASS__, 'save_profile_field' ) );
+	}
+
+	/**
+	 * Whether Twenty Twenty-Five (or a child of it) is active.
+	 *
+	 * @return bool
+	 */
+	public static function is_target_theme() {
+		$theme = wp_get_theme();
+		return 'twentytwentyfive' === $theme->get_template();
+	}
+
+	/**
+	 * Warn in admin if the theme is not supported.
+	 */
+	public static function admin_theme_notice() {
+		if ( self::is_target_theme() || ! current_user_can( 'switch_themes' ) ) {
+			return;
+		}
+
+		$screen = function_exists( 'get_current_screen' ) ? get_current_screen() : null;
+		if ( $screen && 'plugins' !== $screen->id ) {
+			return;
+		}
+
+		echo '<div class="notice notice-warning"><p>';
+		echo esc_html__(
+			'TT5 Light/Dark Toggle is active but Twenty Twenty-Five is not the current theme. The toggle only appears with that theme.',
+			'tt5-light-dark-toggle'
+		);
+		echo '</p></div>';
+	}
+
+	/**
+	 * Prevent flash of wrong theme by applying class before paint.
+	 */
+	public static function inline_preference_script() {
+		$scheme = self::get_initial_scheme();
+		$storage_key = esc_js( TT5_LDT_STORAGE );
+		$cookie_name = esc_js( TT5_LDT_COOKIE );
+
+		?>
+		<script id="tt5-ldt-init">
+		(function () {
+			var storageKey = <?php echo wp_json_encode( TT5_LDT_STORAGE ); ?>;
+			var cookieName = <?php echo wp_json_encode( TT5_LDT_COOKIE ); ?>;
+			var serverScheme = <?php echo wp_json_encode( $scheme ); ?>;
+			var stored = '';
+			try {
+				stored = localStorage.getItem(storageKey) || '';
+			} catch (e) {}
+			if (!stored && document.cookie.indexOf(cookieName + '=') !== -1) {
+				var match = document.cookie.match(new RegExp(cookieName + '=([^;]+)'));
+				stored = match ? decodeURIComponent(match[1]) : '';
+			}
+			var scheme = stored === 'dark' || stored === 'light' ? stored : serverScheme;
+			if (scheme === 'dark') {
+				document.documentElement.classList.add('tt5-ldt-dark');
+			}
+			if (scheme === 'light' || scheme === 'dark') {
+				document.documentElement.setAttribute('data-tt5-scheme', scheme);
+			}
+		})();
+		</script>
+		<?php
+	}
+
+	/**
+	 * Resolve saved scheme for first paint (cookie or user meta only).
+	 *
+	 * @return string light|dark|empty string when none saved.
+	 */
+	public static function get_initial_scheme() {
+		if ( isset( $_COOKIE[ TT5_LDT_COOKIE ] ) ) {
+			$cookie = sanitize_key( wp_unslash( $_COOKIE[ TT5_LDT_COOKIE ] ) );
+			if ( in_array( $cookie, array( 'light', 'dark' ), true ) ) {
+				return $cookie;
+			}
+		}
+
+		if ( is_user_logged_in() ) {
+			$meta = get_user_meta( get_current_user_id(), TT5_LDT_STORAGE, true );
+			if ( in_array( $meta, array( 'light', 'dark' ), true ) ) {
+				return $meta;
+			}
+		}
+
+		return '';
+	}
+
+	/**
+	 * Enqueue styles and toggle script.
+	 */
+	public static function enqueue_assets() {
+		wp_enqueue_style(
+			'tt5-ldt-dark',
+			TT5_LDT_URL . 'assets/css/dark-theme.css',
+			array(),
+			TT5_LDT_VERSION
+		);
+
+		wp_enqueue_style(
+			'tt5-ldt-toggle',
+			TT5_LDT_URL . 'assets/css/toggle.css',
+			array(),
+			TT5_LDT_VERSION
+		);
+
+		wp_enqueue_script(
+			'tt5-ldt-toggle',
+			TT5_LDT_URL . 'assets/js/toggle.js',
+			array(),
+			TT5_LDT_VERSION,
+			true
+		);
+
+		wp_localize_script(
+			'tt5-ldt-toggle',
+			'tt5Ldt',
+			array(
+				'ajaxUrl'    => admin_url( 'admin-ajax.php' ),
+				'nonce'      => wp_create_nonce( 'tt5_ldt_save' ),
+				'storageKey' => TT5_LDT_STORAGE,
+				'cookieName' => TT5_LDT_COOKIE,
+				'cookiePath' => COOKIEPATH ? COOKIEPATH : '/',
+				'cookieDomain' => COOKIE_DOMAIN,
+				'isLoggedIn' => is_user_logged_in(),
+				'i18n'       => array(
+					'light'       => __( 'Switch to light mode', 'tt5-light-dark-toggle' ),
+					'dark'        => __( 'Switch to dark mode', 'tt5-light-dark-toggle' ),
+					'lightLabel'  => __( 'Light mode', 'tt5-light-dark-toggle' ),
+					'darkLabel'   => __( 'Dark mode', 'tt5-light-dark-toggle' ),
+				),
+			)
+		);
+	}
+
+	/**
+	 * Output floating toggle button.
+	 */
+	public static function render_toggle() {
+		$saved   = self::get_initial_scheme();
+		$is_dark = 'dark' === $saved;
+		?>
+		<button
+			type="button"
+			id="tt5-ldt-toggle"
+			class="tt5-ldt-toggle"
+			aria-pressed="<?php echo $is_dark ? 'true' : 'false'; ?>"
+			aria-label="<?php echo esc_attr( $is_dark ? __( 'Switch to light mode', 'tt5-light-dark-toggle' ) : __( 'Switch to dark mode', 'tt5-light-dark-toggle' ) ); ?>"
+			title="<?php echo esc_attr( $is_dark ? __( 'Light mode', 'tt5-light-dark-toggle' ) : __( 'Dark mode', 'tt5-light-dark-toggle' ) ); ?>"
+		>
+			<span class="tt5-ldt-toggle__icon tt5-ldt-toggle__icon--sun" aria-hidden="true">☀</span>
+			<span class="tt5-ldt-toggle__icon tt5-ldt-toggle__icon--moon" aria-hidden="true">☾</span>
+			<span class="tt5-ldt-toggle__label"><?php echo esc_html( $is_dark ? __( 'Light', 'tt5-light-dark-toggle' ) : __( 'Dark', 'tt5-light-dark-toggle' ) ); ?></span>
+		</button>
+		<?php
+	}
+
+	/**
+	 * Persist preference for logged-in users.
+	 */
+	public static function ajax_save_scheme() {
+		check_ajax_referer( 'tt5_ldt_save', 'nonce' );
+
+		$scheme = isset( $_POST['scheme'] ) ? sanitize_key( wp_unslash( $_POST['scheme'] ) ) : '';
+		if ( ! in_array( $scheme, array( 'light', 'dark' ), true ) ) {
+			wp_send_json_error( array( 'message' => 'Invalid scheme' ), 400 );
+		}
+
+		if ( is_user_logged_in() ) {
+			update_user_meta( get_current_user_id(), TT5_LDT_STORAGE, $scheme );
+		}
+
+		wp_send_json_success();
+	}
+
+	/**
+	 * Optional profile field (read-only info).
+	 *
+	 * @param WP_User $user User object.
+	 */
+	public static function render_profile_field( $user ) {
+		$value = get_user_meta( $user->ID, TT5_LDT_STORAGE, true );
+		if ( ! $value ) {
+			return;
+		}
+		?>
+		<h2><?php esc_html_e( 'Color scheme preference', 'tt5-light-dark-toggle' ); ?></h2>
+		<table class="form-table" role="presentation">
+			<tr>
+				<th><label><?php esc_html_e( 'Site appearance', 'tt5-light-dark-toggle' ); ?></label></th>
+				<td>
+					<?php echo esc_html( 'dark' === $value ? __( 'Dark mode', 'tt5-light-dark-toggle' ) : __( 'Light mode', 'tt5-light-dark-toggle' ) ); ?>
+					<p class="description"><?php esc_html_e( 'Saved when you use the front-end light/dark toggle.', 'tt5-light-dark-toggle' ); ?></p>
+				</td>
+			</tr>
+		</table>
+		<?php
+	}
+
+	/**
+	 * Profile save noop (preference saved via AJAX).
+	 *
+	 * @param int $user_id User ID.
+	 */
+	public static function save_profile_field( $user_id ) {
+		if ( ! current_user_can( 'edit_user', $user_id ) ) {
+			return;
+		}
+	}
+}
diff --git a/tt5-light-dark-toggle.php b/tt5-light-dark-toggle.php
new file mode 100644
index 0000000..855a776
--- /dev/null
+++ b/tt5-light-dark-toggle.php
@@ -0,0 +1,28 @@
+<?php
+/**
+ * Plugin Name:       TT5 Light/Dark Toggle
+ * Plugin URI:        https://github.com/example/tt5-light-dark-toggle
+ * Description:       Adds a light/dark mode toggle for the Twenty Twenty-Five WordPress theme.
+ * Version:           1.0.0
+ * Requires at least: 6.4
+ * Requires PHP:      7.4
+ * Author:            TT5 Light/Dark Toggle
+ * License:           GPL-2.0-or-later
+ * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
+ * Text Domain:       tt5-light-dark-toggle
+ *
+ * @package TT5_Light_Dark_Toggle
+ */
+
+defined( 'ABSPATH' ) || exit;
+
+define( 'TT5_LDT_VERSION', '1.0.0' );
+define( 'TT5_LDT_FILE', __FILE__ );
+define( 'TT5_LDT_DIR', plugin_dir_path( __FILE__ ) );
+define( 'TT5_LDT_URL', plugin_dir_url( __FILE__ ) );
+define( 'TT5_LDT_COOKIE', 'tt5_ldt_scheme' );
+define( 'TT5_LDT_STORAGE', 'tt5_ldt_scheme' );
+
+require_once TT5_LDT_DIR . 'includes/class-tt5-ldt.php';
+
+TT5_LDT::init();