diff --git a/views/partials/theme.php b/views/partials/theme.php
new file mode 100644
index 0000000..fc29c2e
--- /dev/null
+++ b/views/partials/theme.php
@@ -0,0 +1,209 @@
+<?php
+$themeSection = $themeSection ?? null;
+
+if ($themeSection === 'head'):
+?>
+<script>
+    (function () {
+        function readThemeCookie() {
+            var cookieParts = document.cookie ? document.cookie.split(';') : [];
+
+            for (var i = 0; i < cookieParts.length; i++) {
+                var part = cookieParts[i].trim();
+                if (part.indexOf('theme=') === 0) {
+                    var value = decodeURIComponent(part.substring(6));
+                    if (value === 'dark' || value === 'light') {
+                        return value;
+                    }
+                    return null;
+                }
+            }
+
+            return null;
+        }
+
+        function resolveTheme() {
+            var cookieTheme = readThemeCookie();
+            if (cookieTheme) {
+                return cookieTheme;
+            }
+
+            if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
+                return 'dark';
+            }
+
+            return 'light';
+        }
+
+        var resolvedTheme = resolveTheme();
+        var html = document.documentElement;
+
+        html.classList.toggle('dark', resolvedTheme === 'dark');
+        html.setAttribute('data-theme', resolvedTheme);
+    })();
+</script>
+
+<style>
+    :root {
+        --theme-bg: #f3f4f6;
+        --theme-surface: #ffffff;
+        --theme-surface-alt: #f9fafb;
+        --theme-text: #111827;
+        --theme-text-muted: #6b7280;
+        --theme-border: #d1d5db;
+    }
+
+    html.dark {
+        --theme-bg: #111827;
+        --theme-surface: #1f2937;
+        --theme-surface-alt: #111827;
+        --theme-text: #f3f4f6;
+        --theme-text-muted: #9ca3af;
+        --theme-border: #374151;
+    }
+
+    .theme-page {
+        background-color: var(--theme-bg);
+        color: var(--theme-text);
+    }
+
+    .theme-surface {
+        background-color: var(--theme-surface);
+    }
+
+    .theme-surface-alt {
+        background-color: var(--theme-surface-alt);
+    }
+
+    .theme-text {
+        color: var(--theme-text);
+    }
+
+    .theme-text-muted {
+        color: var(--theme-text-muted);
+    }
+
+    .theme-border {
+        border-color: var(--theme-border);
+    }
+
+    .theme-input {
+        background-color: var(--theme-surface);
+        border-color: var(--theme-border);
+        color: var(--theme-text);
+    }
+
+    .theme-input::placeholder {
+        color: var(--theme-text-muted);
+    }
+
+    #themeToggle {
+        position: fixed;
+        top: 1rem;
+        right: 1rem;
+        z-index: 50;
+    }
+
+    #themeToggle:focus-visible {
+        outline: 3px solid #2563eb;
+        outline-offset: 2px;
+    }
+</style>
+<?php
+endif;
+
+if ($themeSection === 'body'):
+?>
+<button
+    id="themeToggle"
+    type="button"
+    class="bg-blue-600 hover:bg-blue-700 text-white p-3 rounded-full shadow-md transition"
+    aria-label="Switch to dark mode"
+    title="Switch to dark mode"
+>
+    <i id="themeToggleIcon" class="bi bi-moon-stars-fill" aria-hidden="true"></i>
+</button>
+
+<noscript>
+    <style>
+        #themeToggle {
+            display: none !important;
+        }
+    </style>
+</noscript>
+
+<script>
+    (function () {
+        var html = document.documentElement;
+        var toggleButton = document.getElementById('themeToggle');
+        var toggleIcon = document.getElementById('themeToggleIcon');
+
+        if (!toggleButton || !toggleIcon) {
+            return;
+        }
+
+        function readThemeCookie() {
+            var cookieParts = document.cookie ? document.cookie.split(';') : [];
+
+            for (var i = 0; i < cookieParts.length; i++) {
+                var part = cookieParts[i].trim();
+                if (part.indexOf('theme=') === 0) {
+                    var value = decodeURIComponent(part.substring(6));
+                    if (value === 'dark' || value === 'light') {
+                        return value;
+                    }
+                    return null;
+                }
+            }
+
+            return null;
+        }
+
+        function writeThemeCookie(theme) {
+            document.cookie = 'theme=' + encodeURIComponent(theme) + '; max-age=31536000; path=/; SameSite=Lax';
+        }
+
+        function applyTheme(theme) {
+            var validTheme = theme === 'dark' ? 'dark' : 'light';
+            html.classList.toggle('dark', validTheme === 'dark');
+            html.setAttribute('data-theme', validTheme);
+            return validTheme;
+        }
+
+        function syncToggle(theme) {
+            var isDark = theme === 'dark';
+            var label = isDark ? 'Switch to light mode' : 'Switch to dark mode';
+            var iconClass = isDark ? 'bi-sun-fill' : 'bi-moon-stars-fill';
+
+            toggleButton.setAttribute('aria-label', label);
+            toggleButton.setAttribute('title', label);
+            toggleIcon.className = 'bi ' + iconClass;
+        }
+
+        var initialTheme;
+        if (html.classList.contains('dark')) {
+            initialTheme = 'dark';
+        } else {
+            initialTheme = readThemeCookie() || 'light';
+        }
+
+        var currentTheme = applyTheme(initialTheme);
+        syncToggle(currentTheme);
+
+        toggleButton.addEventListener('click', function () {
+            var nextTheme = currentTheme === 'dark' ? 'light' : 'dark';
+
+            currentTheme = applyTheme(nextTheme);
+
+            try {
+                writeThemeCookie(currentTheme);
+            } catch (error) {
+                // Visual switch remains active if cookie storage is restricted.
+            }
+
+            syncToggle(currentTheme);
+        });
+    })();
+</script>
+<?php
+endif;