diff --git a/frontend/src/app/globals.css b/frontend/src/app/globals.css
index 438ee48..04b0d1c 100644
--- a/frontend/src/app/globals.css
+++ b/frontend/src/app/globals.css
@@ -3,39 +3,93 @@
 
 @custom-variant dark (&:is(.dark *));
 
-@theme {
-  --color-background: #0c0a09;
-  --color-surface: #161311;
-  --color-border: #26201d;
-  /* Claude orange palette (kept under "crimson" token names so all existing classes remap automatically) */
-  --color-crimson: #D97757;
-  --color-crimson-dark: #B8593B;
-  --color-crimson-light: #EA9978;
-  --color-crimson-muted: rgba(217, 119, 87, 0.13);
-
+@theme inline {
+  --color-background: var(--background);
+  --color-surface: var(--surface);
+  --color-border: var(--border);
+  --color-crimson: var(--crimson);
+  --color-crimson-dark: var(--crimson-dark);
+  --color-crimson-light: var(--crimson-light);
+  --color-crimson-muted: var(--crimson-muted);
   --font-sans: var(--font-geist-sans);
   --font-mono: var(--font-geist-mono);
-
   --radius-lg: 0.75rem;
   --radius-md: calc(0.75rem - 2px);
   --radius-sm: calc(0.75rem - 4px);
+  --color-card: var(--card);
+  --color-card-foreground: var(--card-foreground);
+  --color-popover: var(--popover);
+  --color-popover-foreground: var(--popover-foreground);
+  --color-primary: var(--primary);
+  --color-primary-foreground: var(--primary-foreground);
+  --color-secondary: var(--secondary);
+  --color-secondary-foreground: var(--secondary-foreground);
+  --color-muted: var(--muted);
+  --color-muted-foreground: var(--muted-foreground);
+  --color-accent: var(--accent);
+  --color-accent-foreground: var(--accent-foreground);
+  --color-destructive: var(--destructive);
+  --color-input: var(--input);
+  --color-ring: var(--ring);
+  --color-foreground: var(--foreground);
+}
+
+:root {
+  color-scheme: light;
+  --background: #f4efe7;
+  --surface: #fffaf2;
+  --border: #ddd2c4;
+  --crimson: #bf6648;
+  --crimson-dark: #9f4f34;
+  --crimson-light: #d98a6c;
+  --crimson-muted: rgba(191, 102, 72, 0.12);
+  --card: #fffaf2;
+  --card-foreground: #1f1a15;
+  --popover: #fffaf2;
+  --popover-foreground: #1f1a15;
+  --primary: #bf6648;
+  --primary-foreground: #fffaf5;
+  --secondary: #efe6d9;
+  --secondary-foreground: #1f1a15;
+  --muted: #ece3d7;
+  --muted-foreground: #685e53;
+  --accent: #bf6648;
+  --accent-foreground: #fffaf5;
+  --destructive: #dc2626;
+  --input: #fdf8f1;
+  --ring: #bf6648;
+  --foreground: #1f1a15;
+}
+
+.dark {
+  color-scheme: dark;
+  --background: #0c0a09;
+  --surface: #161311;
+  --border: #26201d;
+  --crimson: #d97757;
+  --crimson-dark: #b8593b;
+  --crimson-light: #ea9978;
+  --crimson-muted: rgba(217, 119, 87, 0.13);
+  --card: #161311;
+  --card-foreground: #faf9f7;
+  --popover: #161311;
+  --popover-foreground: #faf9f7;
+  --primary: #d97757;
+  --primary-foreground: #faf9f7;
+  --secondary: #1d1916;
+  --secondary-foreground: #faf9f7;
+  --muted: #211c19;
+  --muted-foreground: #a39990;
+  --accent: #d97757;
+  --accent-foreground: #faf9f7;
+  --destructive: #ef4444;
+  --input: #1d1916;
+  --ring: #d97757;
+  --foreground: #faf9f7;
+}
 
-  --color-card: #161311;
-  --color-card-foreground: #faf9f7;
-  --color-popover: #161311;
-  --color-popover-foreground: #faf9f7;
-  --color-primary: #D97757;
-  --color-primary-foreground: #faf9f7;
-  --color-secondary: #1d1916;
-  --color-secondary-foreground: #faf9f7;
-  --color-muted: #211c19;
-  --color-muted-foreground: #a39990;
-  --color-accent: #D97757;
-  --color-accent-foreground: #faf9f7;
-  --color-destructive: #ef4444;
-  --color-input: #1d1916;
-  --color-ring: #D97757;
-  --color-foreground: #faf9f7;
+html {
+  background-color: var(--background);
 }
 
 * {
@@ -47,6 +101,56 @@ body {
   background-color: var(--color-background);
   color: var(--color-foreground);
   -webkit-font-smoothing: antialiased;
+  transition: background-color 180ms ease, color 180ms ease;
+}
+
+.light :is(div, p, span, h1, h2, h3, h4, li, td, th, pre, code, strong, em, label)[class~="text-white"],
+.light :is(div, p, span, h1, h2, h3, h4, li, td, th, pre, code, strong, em, label)[class~="text-white/90"],
+.light :is(div, p, span, h1, h2, h3, h4, li, td, th, pre, code, strong, em, label)[class~="text-white/85"],
+.light :is(div, p, span, h1, h2, h3, h4, li, td, th, pre, code, strong, em, label)[class~="text-white/80"],
+.light :is(div, p, span, h1, h2, h3, h4, li, td, th, pre, code, strong, em, label)[class~="text-white/75"] {
+  color: var(--color-foreground) !important;
+}
+
+.light :is(div, p, span, li, td, th, pre, code, strong, em, label)[class~="text-white/70"],
+.light :is(div, p, span, li, td, th, pre, code, strong, em, label)[class~="text-white/60"],
+.light :is(div, p, span, li, td, th, pre, code, strong, em, label)[class~="text-white/55"],
+.light :is(div, p, span, li, td, th, pre, code, strong, em, label)[class~="text-white/50"],
+.light :is(div, p, span, li, td, th, pre, code, strong, em, label)[class~="text-white/45"],
+.light :is(div, p, span, li, td, th, pre, code, strong, em, label)[class~="text-white/40"],
+.light :is(div, p, span, li, td, th, pre, code, strong, em, label)[class~="text-white/30"],
+.light :is(div, p, span, li, td, th, pre, code, strong, em, label)[class~="text-white/25"] {
+  color: var(--color-muted-foreground) !important;
+}
+
+.light :is(a, button)[class~="text-white"]:not([class~="bg-crimson"]):not([class~="bg-crimson-dark"]):not([class~="bg-white"]),
+.light :is(a, button)[class~="text-white/85"]:not([class~="bg-crimson"]):not([class~="bg-crimson-dark"]):not([class~="bg-white"]),
+.light :is(a, button)[class~="text-white/75"]:not([class~="bg-crimson"]):not([class~="bg-crimson-dark"]):not([class~="bg-white"]) {
+  color: var(--color-foreground) !important;
+}
+
+.light :is(a, button)[class~="hover:text-white"]:hover {
+  color: var(--color-foreground) !important;
+}
+
+.light :is(a, button)[class~="bg-crimson"][class~="text-white"],
+.light :is(a, button)[class~="bg-crimson-dark"][class~="text-white"] {
+  color: var(--color-primary-foreground) !important;
+}
+
+.light :is(a, button)[class~="bg-white"] {
+  background-color: var(--color-card) !important;
+  color: var(--color-foreground) !important;
+  border: 1px solid var(--color-border) !important;
+}
+
+.light :is(a, button)[class~="bg-white"]:hover,
+.light :is(a, button)[class~="hover:bg-slate-100"]:hover {
+  background-color: var(--color-muted) !important;
+}
+
+.light :is(input, textarea)[class~="placeholder:text-white/40"]::placeholder {
+  color: var(--color-muted-foreground) !important;
 }
 
 .glow-crimson {
diff --git a/frontend/src/app/layout.tsx b/frontend/src/app/layout.tsx
index 6be8a59..e34275e 100644
--- a/frontend/src/app/layout.tsx
+++ b/frontend/src/app/layout.tsx
@@ -2,10 +2,33 @@ import type { Metadata } from "next";
 import { Geist, Geist_Mono } from "next/font/google";
 import "./globals.css";
 import { ProtectedShell } from "@/components/ProtectedShell";
+import { ThemeProvider } from "@/components/ThemeProvider";
+import { ThemeToggle } from "@/components/ThemeToggle";
 
 const geistSans = Geist({ variable: "--font-geist-sans", subsets: ["latin"] });
 const geistMono = Geist_Mono({ variable: "--font-geist-mono", subsets: ["latin"] });
 
+const themeScript = `
+(() => {
+  const storageKey = "osm-theme";
+  try {
+    const stored = window.localStorage.getItem(storageKey);
+    const theme =
+      stored === "light" || stored === "dark"
+        ? stored
+        : (window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light");
+    const root = document.documentElement;
+    root.classList.remove("light", "dark");
+    root.classList.add(theme);
+    root.dataset.theme = theme;
+  } catch {
+    const root = document.documentElement;
+    root.classList.add("dark");
+    root.dataset.theme = "dark";
+  }
+})();
+`;
+
 export const metadata: Metadata = {
   title: "OpenSourceMate",
   description: "Opensource contribution made easier and seamless with AI assistance",
@@ -13,9 +36,17 @@ export const metadata: Metadata = {
 
 export default function RootLayout({ children }: { children: React.ReactNode }) {
   return (
-    <html lang="en" className="dark">
-      <body className={`${geistSans.variable} ${geistMono.variable} antialiased bg-background`}>
-        <ProtectedShell>{children}</ProtectedShell>
+    <html lang="en" suppressHydrationWarning>
+      <head>
+        <script dangerouslySetInnerHTML={{ __html: themeScript }} />
+      </head>
+      <body className={`${geistSans.variable} ${geistMono.variable} antialiased bg-background text-foreground`}>
+        <ThemeProvider>
+          <div className="fixed right-4 top-4 z-[120] sm:right-5 sm:top-5">
+            <ThemeToggle className="bg-background/80 backdrop-blur-md" />
+          </div>
+          <ProtectedShell>{children}</ProtectedShell>
+        </ThemeProvider>
       </body>
     </html>
   );
diff --git a/frontend/src/components/ThemeProvider.tsx b/frontend/src/components/ThemeProvider.tsx
new file mode 100644
index 0000000..7f730a1
--- /dev/null
+++ b/frontend/src/components/ThemeProvider.tsx
@@ -0,0 +1,98 @@
+"use client";
+
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useMemo,
+  useState,
+  useSyncExternalStore,
+  type ReactNode,
+} from "react";
+
+type Theme = "light" | "dark";
+
+interface ThemeContextValue {
+  theme: Theme;
+  mounted: boolean;
+  setTheme: (theme: Theme) => void;
+  toggleTheme: () => void;
+}
+
+const STORAGE_KEY = "osm-theme";
+
+const ThemeContext = createContext<ThemeContextValue | null>(null);
+
+function applyTheme(theme: Theme) {
+  const root = document.documentElement;
+  root.classList.remove("light", "dark");
+  root.classList.add(theme);
+  root.dataset.theme = theme;
+}
+
+function getThemeFromDom(): Theme | null {
+  if (typeof document === "undefined") return null;
+  const domTheme = document.documentElement.dataset.theme;
+  return domTheme === "light" || domTheme === "dark" ? domTheme : null;
+}
+
+function resolvePreferredTheme(): Theme {
+  const domTheme = getThemeFromDom();
+  if (domTheme) return domTheme;
+
+  if (typeof window === "undefined") return "dark";
+
+  const stored = window.localStorage.getItem(STORAGE_KEY);
+  if (stored === "light" || stored === "dark") return stored;
+
+  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+}
+
+export function ThemeProvider({ children }: { children: ReactNode }) {
+  const mounted = useSyncExternalStore(
+    () => () => undefined,
+    () => true,
+    () => false
+  );
+  const [theme, setThemeState] = useState<Theme>(() => resolvePreferredTheme());
+
+  useEffect(() => {
+    if (!mounted) return;
+    window.localStorage.setItem(STORAGE_KEY, theme);
+    applyTheme(theme);
+  }, [mounted, theme]);
+
+  useEffect(() => {
+    if (typeof window === "undefined") return;
+
+    const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
+    const handleChange = (event: MediaQueryListEvent) => {
+      const stored = window.localStorage.getItem(STORAGE_KEY);
+      if (stored === "light" || stored === "dark") return;
+      const nextTheme = event.matches ? "dark" : "light";
+      applyTheme(nextTheme);
+      setThemeState(nextTheme);
+    };
+
+    mediaQuery.addEventListener("change", handleChange);
+    return () => mediaQuery.removeEventListener("change", handleChange);
+  }, []);
+
+  const value = useMemo<ThemeContextValue>(
+    () => ({
+      theme,
+      mounted,
+      setTheme: (nextTheme) => setThemeState(nextTheme),
+      toggleTheme: () => setThemeState((current) => (current === "dark" ? "light" : "dark")),
+    }),
+    [mounted, theme]
+  );
+
+  return <ThemeContext.Provider value={value}>{children}</ThemeContext.Provider>;
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext);
+  if (!context) throw new Error("useTheme must be used inside ThemeProvider");
+  return context;
+}
diff --git a/frontend/src/components/ThemeToggle.tsx b/frontend/src/components/ThemeToggle.tsx
new file mode 100644
index 0000000..beafe83
--- /dev/null
+++ b/frontend/src/components/ThemeToggle.tsx
@@ -0,0 +1,44 @@
+"use client";
+
+import { Moon, Sun } from "lucide-react";
+
+import { useTheme } from "@/components/ThemeProvider";
+import { cn } from "@/lib/utils";
+
+interface ThemeToggleProps {
+  className?: string;
+}
+
+export function ThemeToggle({ className }: ThemeToggleProps) {
+  const { mounted, theme, toggleTheme } = useTheme();
+
+  if (!mounted) {
+    return (
+      <span
+        aria-hidden="true"
+        className={cn(
+          "inline-flex h-10 w-10 shrink-0 rounded-xl border border-border bg-background/70 opacity-0",
+          className
+        )}
+      />
+    );
+  }
+
+  const nextTheme = theme === "dark" ? "light" : "dark";
+
+  return (
+    <button
+      type="button"
+      onClick={toggleTheme}
+      aria-label={`Switch to ${nextTheme} theme`}
+      title={`Switch to ${nextTheme} theme`}
+      className={cn(
+        "inline-flex h-10 w-10 shrink-0 items-center justify-center rounded-xl border border-border bg-background/70 text-muted-foreground shadow-sm transition-all",
+        "hover:border-crimson/35 hover:bg-surface hover:text-foreground",
+        className
+      )}
+    >
+      {theme === "dark" ? <Sun size={18} /> : <Moon size={18} />}
+    </button>
+  );
+}