diff --git a/panel/index.html b/panel/index.html
index 02119e7..8de346c 100644
--- a/panel/index.html
+++ b/panel/index.html
@@ -1,9 +1,20 @@
 <!doctype html>
-<html lang="en" class="dark">
+<html lang="en">
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Felis · Control Panel</title>
+    <script>
+      try {
+        const theme = localStorage.getItem("felis-theme") || "system";
+        const dark = theme === "dark" || (theme === "system" && window.matchMedia("(prefers-color-scheme: dark)").matches);
+        if (dark) {
+          document.documentElement.classList.add("dark");
+        } else {
+          document.documentElement.classList.remove("dark");
+        }
+      } catch (_) {}
+    </script>
   </head>
   <body>
     <div id="root"></div>
diff --git a/panel/src/App.tsx b/panel/src/App.tsx
index b3dced4..fa55429 100644
--- a/panel/src/App.tsx
+++ b/panel/src/App.tsx
@@ -1,4 +1,5 @@
 import { BrowserRouter, Routes, Route, Navigate } from "react-router-dom";
+import { ThemeProvider } from "@/lib/theme";
 import { TierProvider } from "@/lib/tier";
 import { AppShell } from "@/components/AppShell";
 import { RequireAdmin } from "@/components/RequireAdmin";
@@ -23,8 +24,9 @@ import { FleetTable } from "@/pages/ops/FleetTable";
 // the split is by concern, not by tier. TierProvider fetches /me once at boot.
 export default function App() {
   return (
-    <TierProvider>
-      <BrowserRouter>
+    <ThemeProvider storageKey="felis-theme">
+      <TierProvider>
+        <BrowserRouter>
         <Routes>
           {/* Pre-app local-password surfaces (spec §B1). They sit OUTSIDE
               RequireAuth — RequireAuth redirects here — and outside AppShell, so
@@ -62,5 +64,6 @@ export default function App() {
         </Routes>
       </BrowserRouter>
     </TierProvider>
+  </ThemeProvider>
   );
 }
diff --git a/panel/src/components/AppShell.tsx b/panel/src/components/AppShell.tsx
index 46c6c07..22da94f 100644
--- a/panel/src/components/AppShell.tsx
+++ b/panel/src/components/AppShell.tsx
@@ -1,19 +1,20 @@
 import { useEffect } from "react";
 import { NavLink, Outlet } from "react-router-dom";
-import { Cat, Globe } from "lucide-react";
+import { Cat, Globe, Sun, Moon } from "lucide-react";
 import { useTranslation } from "react-i18next";
 import * as SelectPrimitive from "@radix-ui/react-select";
 import { cn } from "@/lib/utils";
 import { useTier } from "@/lib/tier";
 import { visibleSections } from "@/lib/nav";
 import { Select, SelectContent, SelectItem } from "@/components/ui/select";
+import { useTheme } from "@/lib/theme";
 
 function LangToggle() {
   const { i18n } = useTranslation();
 
   return (
     <Select value={i18n.language} onValueChange={(v) => i18n.changeLanguage(v)}>
-      <SelectPrimitive.Trigger className="h-7 w-7 p-0 flex items-center justify-center border border-border rounded-md bg-zinc-900/50 hover:bg-accent text-muted-foreground hover:text-foreground focus:outline-none transition-colors shrink-0">
+      <SelectPrimitive.Trigger className="h-7 w-7 p-0 flex items-center justify-center border border-border rounded-md bg-muted/40 hover:bg-accent text-muted-foreground hover:text-foreground focus:outline-none transition-colors shrink-0">
         <Globe className="h-4 w-4" />
       </SelectPrimitive.Trigger>
       <SelectContent align="start" className="min-w-[6rem]">
@@ -24,6 +25,26 @@ function LangToggle() {
   );
 }
 
+function ThemeToggle() {
+  const { theme, toggleTheme } = useTheme();
+  const { t } = useTranslation("common");
+
+  return (
+    <button
+      onClick={toggleTheme}
+      className="h-7 w-7 p-0 flex items-center justify-center border border-border rounded-md bg-muted/40 hover:bg-accent text-muted-foreground hover:text-foreground focus:outline-none transition-all duration-200 active:scale-95 shrink-0"
+      aria-label={t("toggle_theme")}
+      title={t("toggle_theme")}
+    >
+      {theme === "light" ? (
+        <Sun className="h-4 w-4" />
+      ) : (
+        <Moon className="h-4 w-4" />
+      )}
+    </button>
+  );
+}
+
 export function AppShell() {
   const { isAdmin } = useTier();
   const { t, i18n } = useTranslation("navigation");
@@ -77,8 +98,11 @@ export function AppShell() {
         </nav>
 
         <div className="mt-auto px-3 pt-4 border-t border-border/60">
-          <div className="flex items-start gap-2.5">
-            <LangToggle />
+          <div className="flex items-start gap-2">
+            <div className="flex items-center gap-1.5 shrink-0">
+              <LangToggle />
+              <ThemeToggle />
+            </div>
             <span className="text-[10px] text-muted-foreground/60 leading-tight pt-1">
               {t("common:brand_tagline")}
             </span>
@@ -87,9 +111,15 @@ export function AppShell() {
       </aside>
 
       <div className="flex min-w-0 flex-1 flex-col">
-        <header className="flex h-14 items-center gap-2 border-b border-border px-4 md:hidden">
-          <Cat className="h-5 w-5 text-primary" />
-          <span className="font-semibold">{t("common:brand_name")}</span>
+        <header className="flex h-14 items-center justify-between border-b border-border px-4 md:hidden">
+          <div className="flex items-center gap-2">
+            <Cat className="h-5 w-5 text-primary" />
+            <span className="font-semibold">{t("common:brand_name")}</span>
+          </div>
+          <div className="flex items-center gap-2">
+            <LangToggle />
+            <ThemeToggle />
+          </div>
         </header>
         <main className="flex-1 p-6">
           <Outlet />
diff --git a/panel/src/i18n/resources/en-US/common.json b/panel/src/i18n/resources/en-US/common.json
index 6de3d5f..e2140d7 100644
--- a/panel/src/i18n/resources/en-US/common.json
+++ b/panel/src/i18n/resources/en-US/common.json
@@ -18,5 +18,6 @@
   "page_title": "Felis · Control Panel",
   "lang_en": "EN",
   "lang_zh": "中文",
-  "lang_toggle_hint": "Switch to {{lang}}"
+  "lang_toggle_hint": "Switch to {{lang}}",
+  "toggle_theme": "Toggle theme"
 }
diff --git a/panel/src/i18n/resources/zh-CN/common.json b/panel/src/i18n/resources/zh-CN/common.json
index 3f5fd48..540c6b6 100644
--- a/panel/src/i18n/resources/zh-CN/common.json
+++ b/panel/src/i18n/resources/zh-CN/common.json
@@ -18,5 +18,6 @@
   "page_title": "Felis · 控制面板",
   "lang_en": "EN",
   "lang_zh": "中文",
-  "lang_toggle_hint": "切换至 {{lang}}"
+  "lang_toggle_hint": "切换至 {{lang}}",
+  "toggle_theme": "切换主题"
 }
diff --git a/panel/src/index.css b/panel/src/index.css
index 8afa4fc..e1a7b25 100644
--- a/panel/src/index.css
+++ b/panel/src/index.css
@@ -55,4 +55,10 @@
     font-family: ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
     -webkit-font-smoothing: antialiased;
   }
+
+  /* Global theme toggle transition */
+  .theme-transitioning,
+  .theme-transitioning * {
+    transition: background-color 150ms ease, border-color 150ms ease, color 150ms ease, text-decoration-color 150ms ease, fill 150ms ease, stroke 150ms ease !important;
+  }
 }
diff --git a/panel/src/lib/theme.tsx b/panel/src/lib/theme.tsx
new file mode 100644
index 0000000..a8817c3
--- /dev/null
+++ b/panel/src/lib/theme.tsx
@@ -0,0 +1,98 @@
+import { createContext, useContext, useEffect, useState } from "react";
+
+export type Theme = "light" | "dark";
+
+type ThemeProviderProps = {
+  children: React.ReactNode;
+  storageKey?: string;
+};
+
+type ThemeProviderState = {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+  toggleTheme: () => void;
+};
+
+const initialState: ThemeProviderState = {
+  theme: "dark",
+  setTheme: () => null,
+  toggleTheme: () => null,
+};
+
+const ThemeProviderContext = createContext<ThemeProviderState>(initialState);
+
+export function ThemeProvider({
+  children,
+  storageKey = "felis-theme",
+  ...props
+}: ThemeProviderProps) {
+  const [theme, setThemeState] = useState<Theme>(() => {
+    const stored = localStorage.getItem(storageKey) as Theme | null;
+    if (stored === "light" || stored === "dark") {
+      return stored;
+    }
+    // Auto-detect system preference
+    return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+  });
+
+  useEffect(() => {
+    const root = window.document.documentElement;
+    root.classList.remove("light", "dark");
+    root.classList.add(theme);
+  }, [theme]);
+
+  // Listen to system preference changes ONLY IF the user hasn't explicitly set a preference yet
+  useEffect(() => {
+    const stored = localStorage.getItem(storageKey);
+    if (stored === "light" || stored === "dark") {
+      return;
+    }
+
+    const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
+    const handleSystemThemeChange = (e: MediaQueryListEvent) => {
+      const root = window.document.documentElement;
+      root.classList.add("theme-transitioning");
+      setThemeState(e.matches ? "dark" : "light");
+      setTimeout(() => {
+        root.classList.remove("theme-transitioning");
+      }, 150);
+    };
+
+    mediaQuery.addEventListener("change", handleSystemThemeChange);
+    return () => {
+      mediaQuery.removeEventListener("change", handleSystemThemeChange);
+    };
+  }, [storageKey]);
+
+  const setTheme = (newTheme: Theme) => {
+    const root = window.document.documentElement;
+    root.classList.add("theme-transitioning");
+    localStorage.setItem(storageKey, newTheme);
+    setThemeState(newTheme);
+    setTimeout(() => {
+      root.classList.remove("theme-transitioning");
+    }, 150);
+  };
+
+  const toggleTheme = () => {
+    setTheme(theme === "dark" ? "light" : "dark");
+  };
+
+  return (
+    <ThemeProviderContext.Provider
+      {...props}
+      value={{ theme, setTheme, toggleTheme }}
+    >
+      {children}
+    </ThemeProviderContext.Provider>
+  );
+}
+
+export const useTheme = () => {
+  const context = useContext(ThemeProviderContext);
+
+  if (context === undefined)
+    throw new Error("useTheme must be used within a ThemeProvider");
+
+  return context;
+};