diff --git a/app/static/app.js b/app/static/app.js
index 9265b34..0552c4d 100644
--- a/app/static/app.js
+++ b/app/static/app.js
@@ -1,5 +1,6 @@
 (function () {
   const STORAGE_KEY = "rename_mvp_session_id";
+  const THEME_STORAGE_KEY = "rename_mvp_theme";
 
   const state = {
     sessionId: initSessionId(),
@@ -29,6 +30,7 @@
   const el = {
     sessionMeta: document.getElementById("sessionMeta"),
     outputBox: document.getElementById("outputBox"),
+    themeToggleBtn: document.getElementById("themeToggleBtn"),
     searchInput: document.getElementById("searchInput"),
     searchDropdownBtn: document.getElementById("searchDropdownBtn"),
     searchCombobox: document.getElementById("searchCombobox"),
@@ -92,6 +94,33 @@
     return created;
   }
 
+  function getStoredTheme() {
+    const stored = localStorage.getItem(THEME_STORAGE_KEY);
+    if (stored === "light" || stored === "dark") return stored;
+    return "dark";
+  }
+
+  function applyTheme(theme) {
+    const normalized = theme === "light" ? "light" : "dark";
+    document.documentElement.setAttribute("data-theme", normalized);
+    localStorage.setItem(THEME_STORAGE_KEY, normalized);
+    if (el.themeToggleBtn) {
+      const isDark = normalized === "dark";
+      // Icon represents switch action: dark -> light (sun), light -> dark (moon).
+      el.themeToggleBtn.textContent = isDark ? "☀️" : "🌙";
+      el.themeToggleBtn.setAttribute(
+        "aria-label",
+        isDark ? "Switch to light theme" : "Switch to dark theme"
+      );
+      el.themeToggleBtn.title = isDark ? "Switch to light theme" : "Switch to dark theme";
+    }
+  }
+
+  function toggleTheme() {
+    const current = document.documentElement.getAttribute("data-theme") === "light" ? "light" : "dark";
+    applyTheme(current === "dark" ? "light" : "dark");
+  }
+
   function q(path) {
     return path.includes("?")
       ? `${path}&session_id=${encodeURIComponent(state.sessionId)}`
@@ -834,6 +863,9 @@
   }
 
   function bindEvents() {
+    if (el.themeToggleBtn) {
+      el.themeToggleBtn.addEventListener("click", toggleTheme);
+    }
     el.searchBtn.addEventListener("click", () => withHandler(doSearch, el.searchBtn));
     el.searchInput.addEventListener("focus", openRememberedDropdown);
     el.searchInput.addEventListener("click", openRememberedDropdown);
@@ -903,6 +935,7 @@
   }
 
   async function init() {
+    applyTheme(getStoredTheme());
     el.sessionMeta.textContent = `session_id: ${state.sessionId}`;
     el.modalRecursiveInput.checked = true;
     renderSelectedSeriesDetails();
diff --git a/app/static/styles.css b/app/static/styles.css
index 4afc7c9..011f04d 100644
--- a/app/static/styles.css
+++ b/app/static/styles.css
@@ -2,12 +2,74 @@
   box-sizing: border-box;
 }
 
+:root {
+  --bg-page: #111827;
+  --text-primary: #e5e7eb;
+  --text-muted: #94a3b8;
+  --topbar-bg: #020617;
+  --topbar-text: #e2e8f0;
+  --surface: #1f2937;
+  --surface-elevated: #0f172a;
+  --surface-subtle: #1e293b;
+  --border: #334155;
+  --border-soft: #475569;
+  --divider: #334155;
+  --button-primary-bg: #2563eb;
+  --button-primary-border: #2563eb;
+  --button-primary-text: #ffffff;
+  --button-secondary-bg: #334155;
+  --button-secondary-border: #475569;
+  --button-secondary-text: #e2e8f0;
+  --button-secondary-hover-bg: #3b4a61;
+  --badge-bg: #0b3a6e;
+  --badge-border: #1d4f85;
+  --badge-text: #dbeafe;
+  --list-selected-bg: #1e3a8a;
+  --season-header-bg: #1e293b;
+  --season-header-border: #334155;
+  --danger-text: #f87171;
+  --overlay-bg: rgba(2, 6, 23, 0.7);
+  --shadow-lg: 0 8px 22px rgba(2, 6, 23, 0.45);
+  --series-image-bg: #0b1220;
+}
+
+[data-theme="light"] {
+  --bg-page: #f2f4f8;
+  --text-primary: #1a1f2b;
+  --text-muted: #64748b;
+  --topbar-bg: #0f172a;
+  --topbar-text: #e2e8f0;
+  --surface: #ffffff;
+  --surface-elevated: #ffffff;
+  --surface-subtle: #f8fafc;
+  --border: #d7dee9;
+  --border-soft: #c3cedf;
+  --divider: #e4eaf2;
+  --button-primary-bg: #0f172a;
+  --button-primary-border: #0f172a;
+  --button-primary-text: #ffffff;
+  --button-secondary-bg: #e2e8f0;
+  --button-secondary-border: #c3cedf;
+  --button-secondary-text: #1a1f2b;
+  --button-secondary-hover-bg: #eef2f7;
+  --badge-bg: #dbeafe;
+  --badge-border: #bfdbfe;
+  --badge-text: #0f172a;
+  --list-selected-bg: #e0f2fe;
+  --season-header-bg: #eef2ff;
+  --season-header-border: #dbe4fb;
+  --danger-text: #b91c1c;
+  --overlay-bg: rgba(2, 6, 23, 0.55);
+  --shadow-lg: 0 8px 22px rgba(15, 23, 42, 0.12);
+  --series-image-bg: #f8fafc;
+}
+
 body {
   margin: 0;
   padding: 0;
   font-family: "Segoe UI", Tahoma, sans-serif;
-  background: #f2f4f8;
-  color: #1a1f2b;
+  background: var(--bg-page);
+  color: var(--text-primary);
   height: 100vh;
   overflow: hidden;
   display: flex;
@@ -19,8 +81,8 @@ body {
   justify-content: space-between;
   align-items: center;
   padding: 12px 16px;
-  background: #0f172a;
-  color: #e2e8f0;
+  background: var(--topbar-bg);
+  color: var(--topbar-text);
 }
 
 .topbar h1 {
@@ -32,6 +94,26 @@ body {
   font-size: 12px;
 }
 
+.topbar-right {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+}
+
+.theme-toggle-btn {
+  border: 1px solid var(--button-secondary-border);
+  background: var(--button-secondary-bg);
+  color: var(--button-secondary-text);
+  border-radius: 999px;
+  width: 34px;
+  height: 34px;
+  padding: 0;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  line-height: 1;
+}
+
 .grid {
   display: grid;
   grid-template-columns: repeat(4, minmax(280px, 1fr));
@@ -44,8 +126,8 @@ body {
 }
 
 .panel {
-  background: #ffffff;
-  border: 1px solid #d7dee9;
+  background: var(--surface);
+  border: 1px solid var(--border);
   border-radius: 8px;
   padding: 10px;
   min-height: 420px;
@@ -130,10 +212,10 @@ body {
   right: 0;
   top: calc(100% - 6px);
   z-index: 20;
-  background: #ffffff;
-  border: 1px solid #d7dee9;
+  background: var(--surface-elevated);
+  border: 1px solid var(--border);
   border-radius: 6px;
-  box-shadow: 0 8px 22px rgba(15, 23, 42, 0.12);
+  box-shadow: var(--shadow-lg);
   padding: 6px;
 }
 
@@ -157,7 +239,7 @@ body {
 #rememberedDropdownList .remembered-remove-btn {
   border: none;
   background: transparent;
-  color: #64748b;
+  color: var(--text-muted);
   width: 22px;
   height: 22px;
   line-height: 1;
@@ -168,8 +250,8 @@ body {
 }
 
 #rememberedDropdownList .remembered-remove-btn:hover {
-  background: #eef2f7;
-  color: #0f172a;
+  background: var(--button-secondary-hover-bg);
+  color: var(--text-primary);
 }
 
 .stack {
@@ -186,25 +268,27 @@ body {
 
 input[type="text"],
 select {
-  border: 1px solid #c3cedf;
+  border: 1px solid var(--border-soft);
   border-radius: 6px;
   padding: 6px 8px;
   min-width: 160px;
+  background: var(--surface-subtle);
+  color: var(--text-primary);
 }
 
 button {
-  border: 1px solid #0f172a;
-  background: #0f172a;
-  color: #ffffff;
+  border: 1px solid var(--button-primary-border);
+  background: var(--button-primary-bg);
+  color: var(--button-primary-text);
   border-radius: 6px;
   padding: 6px 10px;
   cursor: pointer;
 }
 
 button.secondary {
-  background: #e2e8f0;
-  color: #1a1f2b;
-  border-color: #c3cedf;
+  background: var(--button-secondary-bg);
+  color: var(--button-secondary-text);
+  border-color: var(--button-secondary-border);
 }
 
 .list {
@@ -213,8 +297,9 @@ button.secondary {
   padding: 0;
   max-height: 260px;
   overflow: auto;
-  border: 1px solid #e4eaf2;
+  border: 1px solid var(--divider);
   border-radius: 6px;
+  background: var(--surface-elevated);
 }
 
 .linked-list-wrap {
@@ -256,22 +341,22 @@ button.secondary {
   display: inline-block;
   font-size: 11px;
   font-weight: 700;
-  color: #0f172a;
-  background: #dbeafe;
-  border: 1px solid #bfdbfe;
+  color: var(--badge-text);
+  background: var(--badge-bg);
+  border: 1px solid var(--badge-border);
   border-radius: 999px;
   padding: 1px 6px;
   margin-right: 6px;
 }
 
 .list li.selected {
-  background: #e0f2fe;
+  background: var(--list-selected-bg);
 }
 
 .list li.season-header {
-  background: #eef2ff;
-  border-bottom: 1px solid #dbe4fb;
-  color: #1e293b;
+  background: var(--season-header-bg);
+  border-bottom: 1px solid var(--season-header-border);
+  color: var(--text-primary);
   font-weight: 700;
   justify-content: flex-start;
   padding: 8px;
@@ -280,8 +365,8 @@ button.secondary {
 .panel-footer {
   position: sticky;
   bottom: 0;
-  background: #ffffff;
-  border-top: 1px solid #e4eaf2;
+  background: var(--surface);
+  border-top: 1px solid var(--divider);
   padding-top: 8px;
   margin-top: 10px;
   display: flex;
@@ -297,14 +382,14 @@ button.secondary {
 }
 
 .mismatch {
-  color: #b91c1c;
+  color: var(--danger-text);
   font-weight: 700;
 }
 
 .modal {
   position: fixed;
   inset: 0;
-  background: rgba(2, 6, 23, 0.55);
+  background: var(--overlay-bg);
   display: flex;
   align-items: center;
   justify-content: center;
@@ -318,8 +403,8 @@ button.secondary {
 .modal-card {
   width: min(1400px, 90vw);
   height: 80vh;
-  background: #ffffff;
-  border: 1px solid #d7dee9;
+  background: var(--surface);
+  border: 1px solid var(--border);
   border-radius: 10px;
   padding: 12px;
   display: flex;
@@ -377,7 +462,7 @@ button.secondary {
   margin-top: 10px;
   margin-bottom: 0;
   justify-content: flex-end;
-  border-top: 1px solid #e4eaf2;
+  border-top: 1px solid var(--divider);
   padding-top: 10px;
 }
 
@@ -388,7 +473,7 @@ button.secondary {
 .settings-section h4 {
   margin: 0 0 8px;
   font-size: 14px;
-  color: #1e293b;
+  color: var(--text-primary);
 }
 
 .settings-field {
@@ -400,7 +485,7 @@ button.secondary {
 
 .settings-field label {
   font-size: 12px;
-  color: #64748b;
+  color: var(--text-muted);
 }
 
 .settings-field select {
@@ -422,7 +507,7 @@ button.secondary {
   align-items: center;
   gap: 8px;
   font-size: 13px;
-  color: #334155;
+  color: var(--text-primary);
 }
 
 .settings-actions {
@@ -434,16 +519,16 @@ button.secondary {
 #panelSelectedEpisodes .panel-footer button:last-child,
 #panelSelectedFiles .panel-footer button:first-child,
 #panelSelectedFiles .panel-footer button:last-child {
-  border-color: #0b3a6e;
-  background: #0b3a6e;
-  color: #ffffff;
+  border-color: var(--button-primary-border);
+  background: var(--button-primary-bg);
+  color: var(--button-primary-text);
 }
 
 .list li {
   display: flex;
   justify-content: space-between;
   gap: 8px;
-  border-bottom: 1px solid #edf1f7;
+  border-bottom: 1px solid var(--divider);
   padding: 6px 8px;
   font-size: 13px;
 }
@@ -474,7 +559,7 @@ button.secondary {
 }
 
 .muted {
-  color: #475569;
+  color: var(--text-muted);
   font-size: 12px;
   margin-bottom: 8px;
 }
@@ -503,7 +588,7 @@ button.secondary {
 }
 
 .series-details {
-  border-top: 1px solid #e4eaf2;
+  border-top: 1px solid var(--divider);
   padding-top: 10px;
 }
 
@@ -522,32 +607,32 @@ button.secondary {
   display: block;
   margin: 0;
   border-radius: 6px;
-  border: 1px solid #d7dee9;
-  background: #f8fafc;
+  border: 1px solid var(--border);
+  background: var(--series-image-bg);
 }
 
 .series-meta {
   font-size: 12px;
-  color: #334155;
+  color: var(--text-primary);
   display: grid;
   gap: 4px;
   margin-bottom: 8px;
 }
 
 .series-meta span:first-child {
-  color: #64748b;
+  color: var(--text-muted);
 }
 
 .series-overview {
   margin: 0 0 8px;
   font-size: 12px;
   line-height: 1.35;
-  color: #1e293b;
+  color: var(--text-primary);
 }
 
 .series-link {
   font-size: 12px;
-  color: #64748b;
+  color: var(--text-muted);
   text-decoration: none;
 }
 
@@ -557,13 +642,14 @@ button.secondary {
 
 #outputBox {
   margin: 0;
-  background: #0b1220;
-  color: #dbeafe;
+  background: var(--surface-subtle);
+  color: var(--text-primary);
   border-radius: 6px;
   padding: 10px;
   max-height: 320px;
   overflow: auto;
   font-size: 12px;
+  border: 1px solid var(--border);
 }
 
 .debug-page {
diff --git a/app/templates/debug.html b/app/templates/debug.html
index ac885e6..f67ad57 100644
--- a/app/templates/debug.html
+++ b/app/templates/debug.html
@@ -4,6 +4,16 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <title>Rename MVP Debug</title>
+    <script>
+      (function () {
+        var key = "rename_mvp_theme";
+        var theme = localStorage.getItem(key);
+        if (theme !== "light" && theme !== "dark") {
+          theme = "dark";
+        }
+        document.documentElement.setAttribute("data-theme", theme);
+      })();
+    </script>
     <link rel="stylesheet" href="/static/styles.css" />
   </head>
   <body>
diff --git a/app/templates/index.html b/app/templates/index.html
index fc9c48d..2af9544 100644
--- a/app/templates/index.html
+++ b/app/templates/index.html
@@ -4,12 +4,25 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <title>Rename MVP</title>
+    <script>
+      (function () {
+        var key = "rename_mvp_theme";
+        var theme = localStorage.getItem(key);
+        if (theme !== "light" && theme !== "dark") {
+          theme = "dark";
+        }
+        document.documentElement.setAttribute("data-theme", theme);
+      })();
+    </script>
     <link rel="stylesheet" href="/static/styles.css" />
   </head>
   <body>
     <header class="topbar">
       <h1>Rename MVP</h1>
-      <div id="sessionMeta"></div>
+      <div class="topbar-right">
+        <div id="sessionMeta"></div>
+        <button id="themeToggleBtn" class="theme-toggle-btn" type="button" aria-label="Toggle theme">☀️</button>
+      </div>
     </header>
 
     <main class="grid">
diff --git a/data/session_state.sqlite3 b/data/session_state.sqlite3
index 63e3fae..eef230f 100644
Binary files a/data/session_state.sqlite3 and b/data/session_state.sqlite3 differ