diff --git a/src/app/globals.css b/src/app/globals.css
index 03ca3bb..0575a62 100644
--- a/src/app/globals.css
+++ b/src/app/globals.css
@@ -15,6 +15,37 @@
     "Helvetica Neue", Arial, sans-serif;
 }
 
+@theme inline {
+  --color-page: var(--surface-page);
+  --color-surface: var(--surface-panel);
+  --color-surface-muted: var(--surface-muted);
+  --color-foreground: var(--text-primary);
+  --color-muted: var(--text-muted);
+  --color-subtle: var(--border-subtle);
+}
+
+:root {
+  color-scheme: light;
+  --surface-page: #f8fafc;
+  --surface-panel: #ffffff;
+  --surface-muted: #f1f5f9;
+  --text-primary: #0f172a;
+  --text-muted: #64748b;
+  --border-subtle: #e2e8f0;
+  --focus-ring: color-mix(in srgb, var(--color-brand-500) 30%, transparent);
+}
+
+:root[data-color-mode="dark"] {
+  color-scheme: dark;
+  --surface-page: #070d19;
+  --surface-panel: #0f172a;
+  --surface-muted: #172033;
+  --text-primary: #f8fafc;
+  --text-muted: #94a3b8;
+  --border-subtle: #263449;
+  --focus-ring: color-mix(in srgb, var(--color-brand-400) 45%, transparent);
+}
+
 /* ── Themes ──────────────────────────────────────────────────────────────────
    Each block overrides the brand scale; selected via data-theme on <html>
    (Admin → Settings → Theme). Register new themes in src/lib/themes.ts. */
@@ -85,12 +116,15 @@
 }
 
 body {
-  @apply bg-slate-50 text-slate-900 antialiased;
+  @apply bg-page text-foreground antialiased;
+  transition:
+    background-color 150ms ease,
+    color 150ms ease;
 }
 
 @layer components {
   .card {
-    @apply rounded-xl border border-slate-200 bg-white shadow-sm;
+    @apply bg-surface border-subtle rounded-xl border shadow-sm;
   }
 
   .btn-primary,
@@ -104,7 +138,7 @@ body {
   }
 
   .btn-secondary {
-    @apply border border-slate-300 bg-white text-slate-700 hover:bg-slate-50;
+    @apply border-subtle bg-surface text-foreground hover:bg-surface-muted border;
   }
 
   .btn-danger {
@@ -112,11 +146,12 @@ body {
   }
 
   .input {
-    @apply block w-full rounded-lg border border-slate-300 bg-white px-3 py-2 text-sm text-slate-900 placeholder:text-slate-400 focus:border-brand-500 focus:outline-2 focus:outline-brand-500/30;
+    @apply border-subtle bg-surface text-foreground placeholder:text-muted focus:border-brand-500 block w-full rounded-lg border px-3 py-2 text-sm focus:outline-2;
+    outline-color: var(--focus-ring);
   }
 
   .label {
-    @apply mb-1.5 block text-sm font-medium text-slate-700;
+    @apply text-foreground mb-1.5 block text-sm font-medium;
   }
 
   .badge {
@@ -128,14 +163,114 @@ body {
   }
 
   .table-base thead th {
-    @apply border-b border-slate-200 px-4 py-3 text-xs font-semibold tracking-wide text-slate-500 uppercase;
+    @apply border-subtle text-muted border-b px-4 py-3 text-xs font-semibold tracking-wide uppercase;
   }
 
   .table-base tbody td {
-    @apply border-b border-slate-100 px-4 py-3 text-slate-700;
+    @apply border-subtle text-foreground border-b px-4 py-3;
   }
 
   .table-base tbody tr:hover {
-    @apply bg-slate-50;
+    @apply bg-surface-muted;
+  }
+}
+
+/* Compatibility while page-level markup migrates to the semantic tokens. */
+:root[data-color-mode="dark"] .bg-white {
+  background-color: var(--surface-panel);
+}
+
+:root[data-color-mode="dark"] .bg-slate-50 {
+  background-color: var(--surface-page);
+}
+
+:root[data-color-mode="dark"] .bg-slate-100,
+:root[data-color-mode="dark"] [class~="hover:bg-slate-50"]:hover,
+:root[data-color-mode="dark"] [class~="hover:bg-slate-100"]:hover {
+  background-color: var(--surface-muted);
+}
+
+:root[data-color-mode="dark"] .text-slate-900,
+:root[data-color-mode="dark"] .text-slate-800,
+:root[data-color-mode="dark"] .text-slate-700,
+:root[data-color-mode="dark"] .text-slate-600 {
+  color: var(--text-primary);
+}
+
+:root[data-color-mode="dark"] .text-slate-500,
+:root[data-color-mode="dark"] .text-slate-400 {
+  color: var(--text-muted);
+}
+
+:root[data-color-mode="dark"] .border-slate-100,
+:root[data-color-mode="dark"] .border-slate-200,
+:root[data-color-mode="dark"] .border-slate-300,
+:root[data-color-mode="dark"] .divide-slate-100 > :not(:last-child),
+:root[data-color-mode="dark"] .divide-slate-200 > :not(:last-child) {
+  border-color: var(--border-subtle);
+}
+
+:root[data-color-mode="dark"] [class~="bg-brand-50"],
+:root[data-color-mode="dark"] [class~="bg-brand-50/50"],
+:root[data-color-mode="dark"] [class~="bg-brand-100"] {
+  background-color: color-mix(
+    in srgb,
+    var(--color-brand-500) 18%,
+    var(--surface-panel)
+  );
+}
+
+:root[data-color-mode="dark"] [class~="bg-green-50"],
+:root[data-color-mode="dark"] [class~="bg-green-100"] {
+  background-color: color-mix(in srgb, #22c55e 16%, var(--surface-panel));
+}
+
+:root[data-color-mode="dark"] [class~="bg-red-50"],
+:root[data-color-mode="dark"] [class~="bg-red-100"] {
+  background-color: color-mix(in srgb, #ef4444 16%, var(--surface-panel));
+}
+
+:root[data-color-mode="dark"] [class~="bg-amber-50"],
+:root[data-color-mode="dark"] [class~="bg-amber-100"] {
+  background-color: color-mix(in srgb, #f59e0b 16%, var(--surface-panel));
+}
+
+:root[data-color-mode="dark"] [class~="bg-blue-50"],
+:root[data-color-mode="dark"] [class~="bg-blue-100"] {
+  background-color: color-mix(in srgb, #3b82f6 16%, var(--surface-panel));
+}
+
+:root[data-color-mode="dark"] .text-green-600,
+:root[data-color-mode="dark"] .text-green-700 {
+  color: #86efac;
+}
+
+:root[data-color-mode="dark"] .text-red-600,
+:root[data-color-mode="dark"] .text-red-700 {
+  color: #fca5a5;
+}
+
+:root[data-color-mode="dark"] .text-amber-700,
+:root[data-color-mode="dark"] .text-amber-800 {
+  color: #fcd34d;
+}
+
+:root[data-color-mode="dark"] .text-blue-700,
+:root[data-color-mode="dark"] .text-blue-800 {
+  color: #93c5fd;
+}
+
+@media print {
+  :root[data-color-mode="dark"] {
+    --surface-page: #ffffff;
+    --surface-panel: #ffffff;
+    --surface-muted: #f1f5f9;
+    --text-primary: #0f172a;
+    --text-muted: #64748b;
+    --border-subtle: #e2e8f0;
+  }
+
+  .color-mode-toggle {
+    display: none;
   }
 }
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index 6624fda..650b79f 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -3,6 +3,7 @@ import "./globals.css";
 import { getSetting } from "@/lib/settings";
 import { getLocale } from "@/lib/i18n";
 import { isTheme } from "@/lib/themes";
+import { ColorModeToggle } from "@/components/color-mode-toggle";
 
 // Every page reads live data (settings, catalog, session), so nothing is
 // statically prerendered — this keeps `next build` from needing a database.
@@ -25,8 +26,22 @@ export default async function RootLayout({
     getLocale(),
   ]);
   return (
-    <html lang={locale} data-theme={isTheme(theme) ? theme : "indigo"}>
-      <body>{children}</body>
+    <html
+      lang={locale}
+      data-theme={isTheme(theme) ? theme : "indigo"}
+      suppressHydrationWarning
+    >
+      <head>
+        <script
+          dangerouslySetInnerHTML={{
+            __html: `(function(){try{var p=localStorage.getItem("openhosting-color-mode");if(p!=="light"&&p!=="dark")p="system";var m=p==="system"?(matchMedia("(prefers-color-scheme: dark)").matches?"dark":"light"):p;document.documentElement.dataset.colorMode=m;document.documentElement.dataset.colorModePreference=p;document.documentElement.style.colorScheme=m}catch(e){}})();`,
+          }}
+        />
+      </head>
+      <body>
+        {children}
+        <ColorModeToggle />
+      </body>
     </html>
   );
 }
diff --git a/src/components/color-mode-toggle.tsx b/src/components/color-mode-toggle.tsx
new file mode 100644
index 0000000..4c3861b
--- /dev/null
+++ b/src/components/color-mode-toggle.tsx
@@ -0,0 +1,75 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import {
+  COLOR_MODE_STORAGE_KEY,
+  isColorModePreference,
+  resolveColorMode,
+  type ColorModePreference,
+} from "@/lib/color-mode";
+
+const LABELS: Record<ColorModePreference, string> = {
+  system: "System",
+  light: "Light",
+  dark: "Dark",
+};
+
+function applyPreference(preference: ColorModePreference): void {
+  const systemPrefersDark = window.matchMedia(
+    "(prefers-color-scheme: dark)",
+  ).matches;
+  const resolved = resolveColorMode(preference, systemPrefersDark);
+  document.documentElement.dataset.colorMode = resolved;
+  document.documentElement.dataset.colorModePreference = preference;
+  document.documentElement.style.colorScheme = resolved;
+}
+
+export function ColorModeToggle() {
+  const [preference, setPreference] =
+    useState<ColorModePreference>("system");
+
+  useEffect(() => {
+    const saved = window.localStorage.getItem(COLOR_MODE_STORAGE_KEY);
+    const initial = isColorModePreference(saved) ? saved : "system";
+    setPreference(initial);
+    applyPreference(initial);
+
+    const media = window.matchMedia("(prefers-color-scheme: dark)");
+    const onSystemChange = () => {
+      if (
+        document.documentElement.dataset.colorModePreference === "system"
+      ) {
+        applyPreference("system");
+      }
+    };
+    media.addEventListener("change", onSystemChange);
+    return () => media.removeEventListener("change", onSystemChange);
+  }, []);
+
+  function updatePreference(next: ColorModePreference): void {
+    setPreference(next);
+    window.localStorage.setItem(COLOR_MODE_STORAGE_KEY, next);
+    applyPreference(next);
+  }
+
+  return (
+    <label className="color-mode-toggle border-subtle bg-surface text-muted fixed right-4 bottom-4 z-50 flex items-center gap-2 rounded-full border px-3 py-2 text-xs shadow-lg">
+      <span aria-hidden="true">◐</span>
+      <span className="sr-only">Color mode</span>
+      <select
+        aria-label="Color mode"
+        className="text-foreground cursor-pointer bg-transparent font-medium outline-none"
+        value={preference}
+        onChange={(event) =>
+          updatePreference(event.target.value as ColorModePreference)
+        }
+      >
+        {Object.entries(LABELS).map(([value, label]) => (
+          <option key={value} value={value}>
+            {label}
+          </option>
+        ))}
+      </select>
+    </label>
+  );
+}
diff --git a/src/lib/color-mode.test.ts b/src/lib/color-mode.test.ts
new file mode 100644
index 0000000..14ff394
--- /dev/null
+++ b/src/lib/color-mode.test.ts
@@ -0,0 +1,23 @@
+import { describe, expect, test } from "vitest";
+import {
+  isColorModePreference,
+  resolveColorMode,
+} from "@/lib/color-mode";
+
+describe("color mode", () => {
+  test("uses the operating-system preference in system mode", () => {
+    expect(resolveColorMode("system", true)).toBe("dark");
+    expect(resolveColorMode("system", false)).toBe("light");
+  });
+
+  test("keeps an explicit user override", () => {
+    expect(resolveColorMode("light", true)).toBe("light");
+    expect(resolveColorMode("dark", false)).toBe("dark");
+  });
+
+  test("rejects invalid persisted values", () => {
+    expect(isColorModePreference("system")).toBe(true);
+    expect(isColorModePreference("sepia")).toBe(false);
+    expect(isColorModePreference(null)).toBe(false);
+  });
+});
diff --git a/src/lib/color-mode.ts b/src/lib/color-mode.ts
new file mode 100644
index 0000000..c4394f9
--- /dev/null
+++ b/src/lib/color-mode.ts
@@ -0,0 +1,23 @@
+export const COLOR_MODE_STORAGE_KEY = "openhosting-color-mode";
+
+export const COLOR_MODE_PREFERENCES = ["system", "light", "dark"] as const;
+
+export type ColorModePreference = (typeof COLOR_MODE_PREFERENCES)[number];
+export type ResolvedColorMode = Exclude<ColorModePreference, "system">;
+
+export function isColorModePreference(
+  value: string | null,
+): value is ColorModePreference {
+  return COLOR_MODE_PREFERENCES.some((preference) => preference === value);
+}
+
+export function resolveColorMode(
+  preference: ColorModePreference,
+  systemPrefersDark: boolean,
+): ResolvedColorMode {
+  return preference === "system"
+    ? systemPrefersDark
+      ? "dark"
+      : "light"
+    : preference;
+}