diff --git a/web/src/components/header/index.tsx b/web/src/components/header/index.tsx
index d18cd8eba1..f83adb132f 100644
--- a/web/src/components/header/index.tsx
+++ b/web/src/components/header/index.tsx
@@ -12,6 +12,8 @@ import {
   Box,
 } from "@mui/material";
 import { ExitToApp, MoreVert } from "@mui/icons-material";
+import Brightness4Icon from "@mui/icons-material/Brightness4"; // moon — switch to dark
+import Brightness7Icon from "@mui/icons-material/Brightness7"; // sun  — switch to light
 import {
   PAGE_PATH_APPLICATIONS,
   PAGE_PATH_DEPLOYMENTS,
@@ -32,11 +34,14 @@ import logo from "~~/assets/logo.svg";
 import NavLink from "./NavLink";
 import { IconOpenNewTab, LogoImage } from "./styles";
 import useAuth from "~/contexts/auth-context/use-auth";
+import { useThemeMode } from "~/theme";
 
 export const APP_HEADER_HEIGHT = 56;
 
 export const Header: FC = memo(function Header() {
   const { me } = useAuth();
+  const { mode, toggleTheme } = useThemeMode();
+
   const [userAnchorEl, setUserAnchorEl] = useState<HTMLButtonElement | null>(
     null
   );
@@ -127,16 +132,28 @@ export const Header: FC = memo(function Header() {
               >
                 <MoreVert />
               </IconButton>
+
+              {/* Dark / light mode toggle — shows moon in light mode, sun in dark mode */}
+              <IconButton
+                color="inherit"
+                aria-label="Toggle dark mode"
+                onClick={toggleTheme}
+                size="small"
+                sx={{ ml: 1 }}
+              >
+                {mode === "dark" ? <Brightness7Icon /> : <Brightness4Icon />}
+              </IconButton>
+
               <Button
                 aria-label="User Menu"
                 aria-controls="user-menu"
                 aria-haspopup="true"
                 onClick={(e) => setUserAnchorEl(e.currentTarget)}
-                style={{
+                color="inherit"
+                sx={{
                   display: "flex",
                   alignItems: "center",
                   textTransform: "none",
-                  color: "white",
                 }}
               >
                 <Avatar
@@ -147,9 +164,21 @@ export const Header: FC = memo(function Header() {
               </Button>
             </>
           ) : (
-            <NavLink href={PAGE_PATH_LOGIN} active={false}>
-              <Typography variant="body2">Login</Typography>
-            </NavLink>
+            <>
+              {/* Show the toggle even on the login page so unauthenticated users can still switch */}
+              <IconButton
+                color="inherit"
+                aria-label="Toggle dark mode"
+                onClick={toggleTheme}
+                size="small"
+                sx={{ mr: 1 }}
+              >
+                {mode === "dark" ? <Brightness7Icon /> : <Brightness4Icon />}
+              </IconButton>
+              <NavLink href={PAGE_PATH_LOGIN} active={false}>
+                <Typography variant="body2">Login</Typography>
+              </NavLink>
+            </>
           )}
         </Box>
       </Toolbar>
diff --git a/web/src/index.tsx b/web/src/index.tsx
index 390dd75787..70d9ad2585 100644
--- a/web/src/index.tsx
+++ b/web/src/index.tsx
@@ -1,7 +1,8 @@
 import { ThemeProvider, StyledEngineProvider } from "@mui/material";
 import CssBaseline from "@mui/material/CssBaseline";
 import { render } from "react-dom";
-import { theme } from "./theme";
+import { useMemo, useState } from "react";
+import { createAppTheme, ThemeContext, ThemeMode } from "./theme";
 import { Routes } from "./routes";
 import { BrowserRouter } from "react-router-dom";
 import { setupDayjs } from "./utils/setup-dayjs";
@@ -11,6 +12,53 @@ import { AuthProvider } from "./contexts/auth-context";
 import { ToastProvider } from "./contexts/toast-context/toast-provider";
 import { CommandProvider } from "./contexts/command-context";
 
+// The root component — lives here so we can use React state for the theme toggle.
+function App() {
+  // Pick up whatever the user last chose, or fall back to their OS preference
+  const savedMode = localStorage.getItem("theme_mode") as ThemeMode | null;
+  const prefersDark = window.matchMedia("(prefers-color-scheme: dark)").matches;
+  const [mode, setMode] = useState<ThemeMode>(
+    savedMode ?? (prefersDark ? "dark" : "light")
+  );
+
+  const toggleTheme = () => {
+    setMode((current) => {
+      const next = current === "light" ? "dark" : "light";
+      localStorage.setItem("theme_mode", next); // remember across sessions
+      return next;
+    });
+  };
+
+  // Rebuild the MUI theme only when mode actually changes
+  const appTheme = useMemo(() => createAppTheme(mode), [mode]);
+
+  return (
+    <ThemeContext.Provider value={{ mode, toggleTheme }}>
+      <StyledEngineProvider injectFirst>
+        <ThemeProvider theme={appTheme}>
+          <BrowserRouter
+            future={{
+              v7_startTransition: false,
+              v7_relativeSplatPath: false,
+            }}
+          >
+            <ToastProvider>
+              <QueryClientWrap>
+                <AuthProvider>
+                  <CommandProvider>
+                    <CssBaseline />
+                    <Routes />
+                  </CommandProvider>
+                </AuthProvider>
+              </QueryClientWrap>
+            </ToastProvider>
+          </BrowserRouter>
+        </ThemeProvider>
+      </StyledEngineProvider>
+    </ThemeContext.Provider>
+  );
+}
+
 async function run(): Promise<void> {
   if (process.env.ENABLE_MOCK === "true") {
     // NOTE: Ignore check exists this module, because this module exclude from production build.
@@ -47,27 +95,7 @@ Happy PipeCD-ing 🙌
 
   render(
     <CookiesProvider>
-      <StyledEngineProvider injectFirst>
-        <ThemeProvider theme={theme}>
-          <BrowserRouter
-            future={{
-              v7_startTransition: false,
-              v7_relativeSplatPath: false,
-            }}
-          >
-            <ToastProvider>
-              <QueryClientWrap>
-                <AuthProvider>
-                  <CommandProvider>
-                    <CssBaseline />
-                    <Routes />
-                  </CommandProvider>
-                </AuthProvider>
-              </QueryClientWrap>
-            </ToastProvider>
-          </BrowserRouter>
-        </ThemeProvider>
-      </StyledEngineProvider>
+      <App />
     </CookiesProvider>,
     document.getElementById("root")
   );
diff --git a/web/src/theme.ts b/web/src/theme.ts
index c775e47573..a8fff33f9f 100644
--- a/web/src/theme.ts
+++ b/web/src/theme.ts
@@ -1,5 +1,6 @@
 import { createTheme } from "@mui/material/styles";
 import { cyan } from "@mui/material/colors";
+import { createContext, useContext } from "react";
 
 declare module "@mui/material/styles/createTypography" {
   interface FontStyle {
@@ -11,74 +12,97 @@ declare module "@mui/material/styles/createTypography" {
   }
 }
 
-export const theme = createTheme({
-  components: {
-    MuiButtonBase: {
-      defaultProps: {
-        disableRipple: true,
-      },
-    },
-    MuiTypography: {
-      defaultProps: {
-        variantMapping: {
-          body1: "div",
-          body2: "div",
+// The two modes the app can be in
+export type ThemeMode = "light" | "dark";
+
+// A simple context so any component can read the mode and call toggleTheme
+export const ThemeContext = createContext<{
+  mode: ThemeMode;
+  toggleTheme: () => void;
+}>({
+  mode: "light",
+  toggleTheme: () => {},
+});
+
+// Convenience hook — just call useThemeMode() anywhere you need the toggle
+export const useThemeMode = () => useContext(ThemeContext);
+
+// Build a MUI theme for whichever mode is requested.
+// All existing colors and component tweaks are preserved exactly as before.
+export const createAppTheme = (mode: ThemeMode) =>
+  createTheme({
+    components: {
+      MuiButtonBase: {
+        defaultProps: {
+          disableRipple: true,
         },
       },
-    },
-    MuiCssBaseline: {
-      styleOverrides: {
-        html: {
-          height: "100%",
-        },
-        body: {
-          height: "100%",
+      MuiTypography: {
+        defaultProps: {
+          variantMapping: {
+            body1: "div",
+            body2: "div",
+          },
         },
-        "#root": {
-          height: "100%",
-          display: "flex",
-          flexDirection: "column",
-          overflow: "hidden",
+      },
+      MuiCssBaseline: {
+        styleOverrides: {
+          html: {
+            height: "100%",
+          },
+          body: {
+            height: "100%",
+          },
+          "#root": {
+            height: "100%",
+            display: "flex",
+            flexDirection: "column",
+            overflow: "hidden",
+          },
         },
       },
-    },
-    MuiDialog: {
-      styleOverrides: {
-        paper: {
-          borderRadius: 16,
+      MuiDialog: {
+        styleOverrides: {
+          paper: {
+            borderRadius: 16,
+          },
         },
       },
-    },
-    MuiDialogActions: {
-      styleOverrides: {
-        spacing: {
-          padding: 16,
+      MuiDialogActions: {
+        styleOverrides: {
+          spacing: {
+            padding: 16,
+          },
         },
       },
     },
-  },
-  palette: {
-    primary: { main: "#283778" },
-    success: {
-      main: "#539d56",
-      light: "#83cf84",
-      dark: "#216e2b",
-    },
-    error: {
-      main: "#d6442c",
-      light: "#ff7657",
-      dark: "#9d0001",
-    },
-    secondary: cyan,
-    background: {
-      default: "#fafafa",
+    palette: {
+      mode, // this single line is what makes MUI flip all its colours
+      primary: { main: "#283778" },
+      success: {
+        main: "#539d56",
+        light: "#83cf84",
+        dark: "#216e2b",
+      },
+      error: {
+        main: "#d6442c",
+        light: "#ff7657",
+        dark: "#9d0001",
+      },
+      secondary: cyan,
+      background: {
+        default: mode === "light" ? "#fafafa" : "#121212",
+        paper: mode === "light" ? "#ffffff" : "#1e1e1e",
+      },
     },
-  },
-  typography: {
-    subtitle2: {
-      fontWeight: 600,
+    typography: {
+      subtitle2: {
+        fontWeight: 600,
+      },
+      fontFamilyMono:
+        '"Roboto Mono",SFMono-Regular,Menlo,Monaco,Consolas,"Liberation Mono","Courier New",monospace',
     },
-    fontFamilyMono:
-      '"Roboto Mono",SFMono-Regular,Menlo,Monaco,Consolas,"Liberation Mono","Courier New",monospace',
-  },
-});
+  });
+
+// Keep the old named export so nothing else in the project breaks
+export const theme = createAppTheme("light");