Docs/Tasks.md

@@ -433,7 +433,9 @@ const source = timeRange === "24h" ? "fail2ban" : "archive";
 
 ---
 
-### TASK-022 — Add dark mode support and OS preference detection
+### TASK-022 — Add dark mode support and OS preference detection (done)
+
+**Where fixed:** `frontend/src/App.tsx`, `frontend/src/providers/ThemeProvider.tsx`, `frontend/src/layouts/MainLayout.tsx`, `frontend/src/theme/customTheme.ts`, `frontend/src/components/BanTrendChart.tsx`, `frontend/src/components/TopCountriesPieChart.tsx`, `frontend/src/components/TopCountriesBarChart.tsx`, `frontend/src/components/JailDistributionChart.tsx`, `Docs/Web-Design.md`
 
 **Where found:** `frontend/src/App.tsx` line 25 — `<FluentProvider theme={lightTheme}>` is hardcoded. `frontend/src/theme/customTheme.ts` already exports `darkTheme` but it is never used.
 

Docs/Web-Design.md

@@ -41,6 +41,7 @@ BanGUI uses a **single custom theme** generated with the [Fluent UI Theme Design
 
 - The primary colour must have a **contrast ratio of at least 4.5 : 1** against `white` for text and **3 : 1** for large text and UI elements.
 - Provide a **dark theme variant** alongside the default light theme. Both must share the same semantic slot names — only the palette values differ.
+- Persist the user's explicit theme choice in `localStorage` and otherwise follow the operating system's `prefers-color-scheme` setting.
 - Never reference Fluent UI palette slots (`themeDarker`, `neutralLight`, etc.) directly in components. Always go through semantic slots so theme switching works seamlessly.
 
 ### Colour Rules

frontend/src/App.tsx

@@ -22,8 +22,9 @@
 import { lazy, Suspense } from "react";
 import { FluentProvider, Spinner } from "@fluentui/react-components";
 import { BrowserRouter, Navigate, Route, Routes } from "react-router-dom";
-import { lightTheme } from "./theme/customTheme";
+import { darkTheme, lightTheme } from "./theme/customTheme";
 import { AuthProvider } from "./providers/AuthProvider";
+import { ThemeProvider, useThemeMode } from "./providers/ThemeProvider";
 import { TimezoneProvider } from "./providers/TimezoneProvider";
 import { RequireAuth } from "./components/RequireAuth";
 import { SetupGuard } from "./components/SetupGuard";
@@ -43,9 +44,12 @@ const BlocklistsPage = lazy(() => import("./pages/BlocklistsPage").then((m) => (
 /**
  * Root application component — mounts providers and top-level routes.
  */
-function App(): React.JSX.Element {
+function AppContents(): React.JSX.Element {
+  const { colorMode } = useThemeMode();
+  const theme = colorMode === "dark" ? darkTheme : lightTheme;
+
   return (
-    <FluentProvider theme={lightTheme}>
+    <FluentProvider theme={theme}>
       <ErrorBoundary>
         <BrowserRouter future={{ v7_startTransition: true, v7_relativeSplatPath: true }}>
           <Suspense fallback={<Spinner size="large" label="Loading…" />}>
@@ -96,4 +100,12 @@ function App(): React.JSX.Element {
   );
 }
 
+function App(): React.JSX.Element {
+  return (
+    <ThemeProvider>
+      <AppContents />
+    </ThemeProvider>
+  );
+}
+
 export default App;

frontend/src/components/BanTrendChart.tsx

@@ -25,6 +25,7 @@ import {
   CHART_PALETTE,
   resolveFluentToken,
 } from "../utils/chartTheme";
+import { useThemeMode } from "../providers/ThemeProvider";
 import { ChartStateWrapper } from "./ChartStateWrapper";
 import { useBanTrend } from "../hooks/useBanTrend";
 import type { BanOriginFilter, TimeRange } from "../types/ban";
@@ -200,29 +201,36 @@ export const BanTrendChart = memo(function BanTrendChart({
   source = "fail2ban",
 }: BanTrendChartProps): React.JSX.Element {
   const styles = useStyles();
+  const { colorMode } = useThemeMode();
   const { buckets, loading, error, reload } = useBanTrend(timeRange, origin, source);
 
   const isEmpty = buckets.every((b) => b.count === 0);
   const entries = buildEntries(buckets, timeRange);
   const { primaryColour, axisColour, gridColour } = useMemo(
-    () => ({
-      primaryColour: resolveFluentToken(CHART_PALETTE[0] ?? ""),
-      axisColour: resolveFluentToken(CHART_AXIS_TEXT_TOKEN),
-      gridColour: resolveFluentToken(CHART_GRID_LINE_TOKEN),
-    }),
-    [],
+    () => {
+      void colorMode;
+      return {
+        primaryColour: resolveFluentToken(CHART_PALETTE[0] ?? ""),
+        axisColour: resolveFluentToken(CHART_AXIS_TEXT_TOKEN),
+        gridColour: resolveFluentToken(CHART_GRID_LINE_TOKEN),
+      };
+    },
+    [colorMode],
   );
   const tickInterval = TICK_INTERVAL[timeRange];
 
   const tooltipContent = useMemo(
-    () => (
-      <TrendTooltip
-        backgroundColor={resolveFluentToken(tokens.colorNeutralBackground1)}
-        borderColor={resolveFluentToken(tokens.colorNeutralStroke2)}
-        textColor={resolveFluentToken(tokens.colorNeutralForeground1)}
-      />
-    ),
-    [],
+    () => {
+      void colorMode;
+      return (
+        <TrendTooltip
+          backgroundColor={resolveFluentToken(tokens.colorNeutralBackground1)}
+          borderColor={resolveFluentToken(tokens.colorNeutralStroke2)}
+          textColor={resolveFluentToken(tokens.colorNeutralForeground1)}
+        />
+      );
+    },
+    [colorMode],
   );
 
   return (

frontend/src/components/JailDistributionChart.tsx

@@ -24,6 +24,7 @@ import {
   CHART_PALETTE,
   resolveFluentToken,
 } from "../utils/chartTheme";
+import { useThemeMode } from "../providers/ThemeProvider";
 import { ChartStateWrapper } from "./ChartStateWrapper";
 import { ChartTooltip } from "./ChartTooltip";
 import { useJailDistribution } from "../hooks/useJailDistribution";
@@ -134,17 +135,21 @@ export const JailDistributionChart = memo(function JailDistributionChart({
   origin,
 }: JailDistributionChartProps): React.JSX.Element {
   const styles = useStyles();
+  const { colorMode } = useThemeMode();
   const { jails, loading, error, reload } = useJailDistribution(timeRange, origin);
 
   const entries = buildEntries(jails);
   const chartHeight = Math.max(entries.length * BAR_HEIGHT_PX, MIN_CHART_HEIGHT);
   const { primaryColour, axisColour, gridColour } = useMemo(
-    () => ({
-      primaryColour: resolveFluentToken(CHART_PALETTE[0] ?? ""),
-      axisColour: resolveFluentToken(CHART_AXIS_TEXT_TOKEN),
-      gridColour: resolveFluentToken(CHART_GRID_LINE_TOKEN),
-    }),
-    [],
+    () => {
+      void colorMode;
+      return {
+        primaryColour: resolveFluentToken(CHART_PALETTE[0] ?? ""),
+        axisColour: resolveFluentToken(CHART_AXIS_TEXT_TOKEN),
+        gridColour: resolveFluentToken(CHART_GRID_LINE_TOKEN),
+      };
+    },
+    [colorMode],
   );
 
   return (

frontend/src/components/TopCountriesBarChart.tsx

@@ -21,6 +21,7 @@ import {
   CHART_GRID_LINE_TOKEN,
   resolveFluentToken,
 } from "../utils/chartTheme";
+import { useThemeMode } from "../providers/ThemeProvider";
 import { ChartTooltip } from "./ChartTooltip";
 
 // ---------------------------------------------------------------------------
@@ -142,16 +143,20 @@ export const TopCountriesBarChart = memo(function TopCountriesBarChart({
   countryNames,
 }: TopCountriesBarChartProps): React.JSX.Element {
   const styles = useStyles();
+  const { colorMode } = useThemeMode();
 
   const entries = buildEntries(countries, countryNames);
 
   const { primaryColour, axisColour, gridColour } = useMemo(
-    () => ({
-      primaryColour: resolveFluentToken(CHART_PALETTE[0] ?? ""),
-      axisColour: resolveFluentToken(CHART_AXIS_TEXT_TOKEN),
-      gridColour: resolveFluentToken(CHART_GRID_LINE_TOKEN),
-    }),
-    [],
+    () => {
+      void colorMode;
+      return {
+        primaryColour: resolveFluentToken(CHART_PALETTE[0] ?? ""),
+        axisColour: resolveFluentToken(CHART_AXIS_TEXT_TOKEN),
+        gridColour: resolveFluentToken(CHART_GRID_LINE_TOKEN),
+      };
+    },
+    [colorMode],
   );
 
   if (entries.length === 0) {

frontend/src/components/TopCountriesPieChart.tsx

@@ -17,6 +17,7 @@ import type { LegendPayload } from "recharts/types/component/DefaultLegendConten
 import type { TooltipContentProps } from "recharts/types/component/Tooltip";
 import { tokens, makeStyles, Text } from "@fluentui/react-components";
 import { CHART_PALETTE, resolveFluentToken } from "../utils/chartTheme";
+import { useThemeMode } from "../providers/ThemeProvider";
 import { ChartTooltip } from "./ChartTooltip";
 
 // ---------------------------------------------------------------------------
@@ -134,10 +135,14 @@ export const TopCountriesPieChart = memo(function TopCountriesPieChart({
   countryNames,
 }: TopCountriesPieChartProps): React.JSX.Element {
   const styles = useStyles();
+  const { colorMode } = useThemeMode();
 
   const resolvedPalette = useMemo(
-    () => CHART_PALETTE.map(resolveFluentToken),
-    [],
+    () => {
+      void colorMode;
+      return CHART_PALETTE.map(resolveFluentToken);
+    },
+    [colorMode],
   );
   const slices = buildSlices(countries, countryNames, resolvedPalette);
   const total = slices.reduce((sum, s) => sum + s.value, 0);

frontend/src/layouts/MainLayout.tsx

@@ -28,11 +28,14 @@ import {
   ListRegular,
   SignOutRegular,
   NavigationRegular,
+  WeatherMoonRegular,
+  WeatherSunnyRegular,
 } from "@fluentui/react-icons";
 import { NavLink, Outlet, useNavigate } from "react-router-dom";
 import { useAuth } from "../hooks/useAuth";
 import { useServerStatus } from "../hooks/useServerStatus";
 import { useBlocklistStatus } from "../hooks/useBlocklist";
+import { useThemeMode } from "../providers/ThemeProvider";
 
 // ---------------------------------------------------------------------------
 // Styles
@@ -146,11 +149,20 @@ const useStyles = makeStyles({
     padding: tokens.spacingVerticalS,
     flexShrink: 0,
   },
+  footerActions: {
+    display: "flex",
+    flexDirection: "column",
+    gap: tokens.spacingVerticalXS,
+  },
+  themeButton: {
+    width: "100%",
+    justifyContent: "flex-start",
+  },
   logoutButton: {
     width: "100%",
     justifyContent: "flex-start",
   },
-  logoutButtonCollapsed: {
+  sidebarButtonCollapsed: {
     justifyContent: "center",
   },
   versionText: {
@@ -220,6 +232,7 @@ const NAV_ITEMS: NavItem[] = [
 export function MainLayout(): React.JSX.Element {
   const styles = useStyles();
   const { logout } = useAuth();
+  const { colorMode, toggleColorMode } = useThemeMode();
   const navigate = useNavigate();
 
   const readSavedCollapsed = (): boolean => {
@@ -346,31 +359,51 @@ export function MainLayout(): React.JSX.Element {
           })}
         </ul>
 
-        {/* Footer — Logout */}
+        {/* Footer — Theme toggle and logout */}
         <div className={styles.sidebarFooter}>
           {!collapsed && (
             <Text className={styles.versionText}>
               BanGUI
             </Text>
           )}
-          <Tooltip
-            content={collapsed ? "Sign out" : ""}
-            relationship="label"
-            positioning="after"
-          >
-            <Button
-              appearance="subtle"
-              icon={<SignOutRegular />}
-              onClick={() => void handleLogout()}
-              aria-label="Sign out"
-              className={mergeClasses(
-                styles.logoutButton,
-                collapsed && styles.logoutButtonCollapsed,
-              )}
+          <div className={styles.footerActions}>
+            <Tooltip
+              content={collapsed ? (colorMode === "dark" ? "Switch to light mode" : "Switch to dark mode") : ""}
+              relationship="label"
+              positioning="after"
             >
-              {!collapsed && "Sign out"}
-            </Button>
-          </Tooltip>
+              <Button
+                appearance="subtle"
+                icon={colorMode === "dark" ? <WeatherSunnyRegular /> : <WeatherMoonRegular />}
+                onClick={toggleColorMode}
+                aria-label={colorMode === "dark" ? "Switch to light mode" : "Switch to dark mode"}
+                className={mergeClasses(
+                  styles.themeButton,
+                  collapsed && styles.sidebarButtonCollapsed,
+                )}
+              >
+                {!collapsed && (colorMode === "dark" ? "Light mode" : "Dark mode")}
+              </Button>
+            </Tooltip>
+            <Tooltip
+              content={collapsed ? "Sign out" : ""}
+              relationship="label"
+              positioning="after"
+            >
+              <Button
+                appearance="subtle"
+                icon={<SignOutRegular />}
+                onClick={() => void handleLogout()}
+                aria-label="Sign out"
+                className={mergeClasses(
+                  styles.logoutButton,
+                  collapsed && styles.sidebarButtonCollapsed,
+                )}
+              >
+                {!collapsed && "Sign out"}
+              </Button>
+            </Tooltip>
+          </div>
         </div>
       </nav>
 

frontend/src/layouts/__tests__/MainLayout.test.tsx

@@ -89,4 +89,9 @@ describe("MainLayout", () => {
     await userEvent.click(toggleButton);
     expect(localStorage.getItem("bangui_sidebar_collapsed")).toBe("true");
   });
+
+  it("renders a theme toggle button in the sidebar footer", () => {
+    renderLayout();
+    expect(screen.getByRole("button", { name: /switch to dark mode/i })).toBeInTheDocument();
+  });
 });

frontend/src/providers/ThemeProvider.tsx

@@ -0,0 +1,98 @@
+import { createContext, useCallback, useContext, useEffect, useMemo, useState } from "react";
+import type { ReactNode } from "react";
+
+type ThemeMode = "light" | "dark";
+
+interface ThemeProviderContext {
+  colorMode: ThemeMode;
+  toggleColorMode: () => void;
+  setColorMode: (mode: ThemeMode) => void;
+  hasExplicitPreference: boolean;
+}
+
+const THEME_STORAGE_KEY = "bangui_theme";
+const ThemeModeContext = createContext<ThemeProviderContext | null>(null);
+
+function readSavedTheme(): ThemeMode | null {
+  try {
+    const value = localStorage.getItem(THEME_STORAGE_KEY);
+    if (value === "dark" || value === "light") {
+      return value;
+    }
+  } catch {
+    // Ignore localStorage failures.
+  }
+  return null;
+}
+
+function getSystemTheme(): ThemeMode {
+  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+}
+
+interface ThemeProviderProps {
+  children: ReactNode;
+}
+
+export function ThemeProvider({ children }: ThemeProviderProps): React.JSX.Element {
+  const savedTheme = readSavedTheme();
+  const [colorMode, setColorModeState] = useState<ThemeMode>(() => savedTheme ?? getSystemTheme());
+  const [hasExplicitPreference, setHasExplicitPreference] = useState<boolean>(() => savedTheme !== null);
+
+  const setColorMode = useCallback((mode: ThemeMode): void => {
+    try {
+      localStorage.setItem(THEME_STORAGE_KEY, mode);
+    } catch {
+      // Ignore storage failures.
+    }
+    setHasExplicitPreference(true);
+    setColorModeState(mode);
+  }, []);
+
+  const toggleColorMode = useCallback((): void => {
+    setColorMode(colorMode === "dark" ? "light" : "dark");
+  }, [colorMode, setColorMode]);
+
+  useEffect(() => {
+    const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
+    const handleChange = (event: MediaQueryListEvent): void => {
+      if (!hasExplicitPreference) {
+        setColorModeState(event.matches ? "dark" : "light");
+      }
+    };
+
+    if (typeof mediaQuery.addEventListener === "function") {
+      mediaQuery.addEventListener("change", handleChange);
+      function cleanup(): void {
+        mediaQuery.removeEventListener("change", handleChange);
+      }
+      return cleanup;
+    }
+
+    return undefined;
+  }, [hasExplicitPreference]);
+
+  const value = useMemo(
+    () => ({ colorMode, toggleColorMode, setColorMode, hasExplicitPreference }),
+    [colorMode, toggleColorMode, setColorMode, hasExplicitPreference],
+  );
+
+  return <ThemeModeContext.Provider value={value}>{children}</ThemeModeContext.Provider>;
+}
+
+export function useThemeMode(): ThemeProviderContext {
+  const context = useContext(ThemeModeContext);
+  if (context != null) {
+    return context;
+  }
+
+  return {
+    colorMode: "light",
+    toggleColorMode: (): void => {
+      /* no-op when provider is missing */
+    },
+    setColorMode: (): void => {
+      /* no-op when provider is missing */
+    },
+    hasExplicitPreference: false,
+  };
+}

frontend/src/providers/__tests__/ThemeProvider.test.tsx

@@ -0,0 +1,68 @@
+import { describe, expect, it, beforeEach, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { ThemeProvider, useThemeMode } from "../ThemeProvider";
+
+function ThemeConsumer(): React.JSX.Element {
+  const { colorMode, toggleColorMode } = useThemeMode();
+  return (
+    <div>
+      <span data-testid="color-mode">{colorMode}</span>
+      <button type="button" onClick={toggleColorMode}>
+        Toggle
+      </button>
+    </div>
+  );
+}
+
+function renderWithProvider(): void {
+  render(
+    <ThemeProvider>
+      <ThemeConsumer />
+    </ThemeProvider>,
+  );
+}
+
+describe("ThemeProvider", () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+    localStorage.clear();
+    Object.defineProperty(window, "matchMedia", {
+      configurable: true,
+      writable: true,
+      value: vi.fn().mockReturnValue({
+        matches: false,
+        addEventListener: vi.fn(),
+        removeEventListener: vi.fn(),
+      }),
+    });
+  });
+
+  it("uses the saved theme from localStorage when present", () => {
+    localStorage.setItem("bangui_theme", "dark");
+    renderWithProvider();
+    expect(screen.getByTestId("color-mode")).toHaveTextContent("dark");
+  });
+
+  it("falls back to the OS preference when no explicit theme is saved", () => {
+    Object.defineProperty(window, "matchMedia", {
+      configurable: true,
+      writable: true,
+      value: vi.fn().mockReturnValue({
+        matches: true,
+        addEventListener: vi.fn(),
+        removeEventListener: vi.fn(),
+      }),
+    });
+    renderWithProvider();
+    expect(screen.getByTestId("color-mode")).toHaveTextContent("dark");
+  });
+
+  it("persists the user selection to localStorage when toggled", async () => {
+    renderWithProvider();
+    expect(screen.getByTestId("color-mode")).toHaveTextContent("light");
+
+    await screen.getByRole("button", { name: /toggle/i }).click();
+    expect(screen.getByTestId("color-mode")).toHaveTextContent("dark");
+    expect(localStorage.getItem("bangui_theme")).toBe("dark");
+  });
+});