diff --git a/decnet_web/src/App.tsx b/decnet_web/src/App.tsx
index 814d0fa5..bba561b9 100644
--- a/decnet_web/src/App.tsx
+++ b/decnet_web/src/App.tsx
@@ -182,6 +182,17 @@ function App() {
       }
     } catch { /* fall through to default */ }
     document.documentElement.setAttribute('data-accent', accent);
+
+    /* Lab theme persists in sessionStorage so a tab reload keeps the
+     * dev's chosen theme without leaking to other tabs or users. The
+     * production user-facing toggle (localStorage `decnet_theme`)
+     * arrives with the Config-page setting in a later task. */
+    try {
+      const labTheme = sessionStorage.getItem('decnet_theme_lab');
+      if (labTheme === 'light' || labTheme === 'dark') {
+        document.documentElement.setAttribute('data-theme', labTheme);
+      }
+    } catch { /* ignore */ }
   }, []);
 
   const handleLogin = (newToken: string) => setToken(newToken);
diff --git a/decnet_web/src/components/ThemeLab/ThemeLab.css b/decnet_web/src/components/ThemeLab/ThemeLab.css
index c076daa4..0a4cebdc 100644
--- a/decnet_web/src/components/ThemeLab/ThemeLab.css
+++ b/decnet_web/src/components/ThemeLab/ThemeLab.css
@@ -8,6 +8,19 @@
   gap: var(--space-8);
 }
 
+.lab-page-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: flex-end;
+  gap: var(--space-4);
+  flex-wrap: wrap;
+}
+
+.lab-theme-toggle {
+  display: inline-flex;
+  gap: var(--space-2);
+}
+
 .theme-lab .page-header h1 {
   font-size: var(--fs-page);
   letter-spacing: var(--ls-title);
diff --git a/decnet_web/src/components/ThemeLab/ThemeLab.tsx b/decnet_web/src/components/ThemeLab/ThemeLab.tsx
index 3a51dc98..dd3b7a0b 100644
--- a/decnet_web/src/components/ThemeLab/ThemeLab.tsx
+++ b/decnet_web/src/components/ThemeLab/ThemeLab.tsx
@@ -1,4 +1,5 @@
 import React, { useEffect, useState } from 'react';
+import ThemeToggle from './ThemeToggle';
 import './ThemeLab.css';
 
 /* Kitchen-sink theme lab.
@@ -314,13 +315,14 @@ const NetBoxes: React.FC = () => (
 const ThemeLab: React.FC = () => {
   return (
     <div className="fleet-root theme-lab" data-testid="theme-lab">
-      <header className="page-header">
+      <header className="page-header lab-page-header">
         <div className="page-title-group">
           <h1>THEME LAB</h1>
           <span className="page-sub">
             dev only · primitive zoo for theme regression
           </span>
         </div>
+        <ThemeToggle />
       </header>
 
       <Section id="swatches" title="COLOUR TOKENS">
diff --git a/decnet_web/src/components/ThemeLab/ThemeToggle.tsx b/decnet_web/src/components/ThemeLab/ThemeToggle.tsx
new file mode 100644
index 00000000..2a219274
--- /dev/null
+++ b/decnet_web/src/components/ThemeLab/ThemeToggle.tsx
@@ -0,0 +1,57 @@
+import React, { useEffect, useState } from 'react';
+
+/* Dev-scoped Dark/Light theme toggle for the theme lab.
+ *
+ * Flips `document.documentElement.dataset.theme` and persists to
+ * **sessionStorage** intentionally — the lab is a tab-scoped
+ * exploration tool. Global persistence (across reloads, all users)
+ * is the user-facing Config toggle that ships in Task 6. */
+
+export const THEME_SESSION_KEY = 'decnet_theme_lab';
+
+export type Theme = 'dark' | 'light';
+
+export function readLabTheme(): Theme {
+  try {
+    const v = sessionStorage.getItem(THEME_SESSION_KEY);
+    return v === 'light' ? 'light' : 'dark';
+  } catch {
+    return 'dark';
+  }
+}
+
+export function applyTheme(theme: Theme): void {
+  document.documentElement.dataset.theme = theme;
+}
+
+const ThemeToggle: React.FC = () => {
+  const [theme, setTheme] = useState<Theme>(() => readLabTheme());
+
+  useEffect(() => {
+    applyTheme(theme);
+    try { sessionStorage.setItem(THEME_SESSION_KEY, theme); } catch { /* ignore */ }
+  }, [theme]);
+
+  return (
+    <div className="lab-theme-toggle" role="group" aria-label="Theme">
+      <button
+        type="button"
+        className={`btn small ${theme === 'dark' ? '' : 'ghost'}`}
+        onClick={() => setTheme('dark')}
+        aria-pressed={theme === 'dark'}
+      >
+        DARK
+      </button>
+      <button
+        type="button"
+        className={`btn small ${theme === 'light' ? '' : 'ghost'}`}
+        onClick={() => setTheme('light')}
+        aria-pressed={theme === 'light'}
+      >
+        LIGHT
+      </button>
+    </div>
+  );
+};
+
+export default ThemeToggle;
diff --git a/decnet_web/src/components/ThemeLab/__tests__/ThemeToggle.test.tsx b/decnet_web/src/components/ThemeLab/__tests__/ThemeToggle.test.tsx
new file mode 100644
index 00000000..afcfca4f
--- /dev/null
+++ b/decnet_web/src/components/ThemeLab/__tests__/ThemeToggle.test.tsx
@@ -0,0 +1,55 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import ThemeToggle, {
+  THEME_SESSION_KEY,
+  readLabTheme,
+  applyTheme,
+} from '../ThemeToggle';
+
+describe('ThemeToggle', () => {
+  beforeEach(() => {
+    sessionStorage.clear();
+    delete document.documentElement.dataset.theme;
+  });
+
+  it('defaults to dark and applies dark on mount', () => {
+    render(<ThemeToggle />);
+    expect(document.documentElement.dataset.theme).toBe('dark');
+    expect(sessionStorage.getItem(THEME_SESSION_KEY)).toBe('dark');
+    expect(screen.getByRole('button', { name: 'DARK' })).toHaveAttribute(
+      'aria-pressed',
+      'true',
+    );
+  });
+
+  it('flips to light on click and persists to sessionStorage', async () => {
+    const user = userEvent.setup();
+    render(<ThemeToggle />);
+    await user.click(screen.getByRole('button', { name: 'LIGHT' }));
+    expect(document.documentElement.dataset.theme).toBe('light');
+    expect(sessionStorage.getItem(THEME_SESSION_KEY)).toBe('light');
+    expect(screen.getByRole('button', { name: 'LIGHT' })).toHaveAttribute(
+      'aria-pressed',
+      'true',
+    );
+  });
+
+  it('hydrates from existing sessionStorage value', () => {
+    sessionStorage.setItem(THEME_SESSION_KEY, 'light');
+    render(<ThemeToggle />);
+    expect(document.documentElement.dataset.theme).toBe('light');
+  });
+
+  it('readLabTheme falls back to dark on bad value', () => {
+    sessionStorage.setItem(THEME_SESSION_KEY, 'banana');
+    expect(readLabTheme()).toBe('dark');
+  });
+
+  it('applyTheme writes the html attribute', () => {
+    applyTheme('light');
+    expect(document.documentElement.dataset.theme).toBe('light');
+    applyTheme('dark');
+    expect(document.documentElement.dataset.theme).toBe('dark');
+  });
+});
diff --git a/decnet_web/src/index.css b/decnet_web/src/index.css
index a8e53f63..5813c0b5 100644
--- a/decnet_web/src/index.css
+++ b/decnet_web/src/index.css
@@ -108,6 +108,58 @@ html[data-accent="violet"] {
   --accent-glow:    var(--violet-glow);
 }
 
+/* ── Light theme ────────────────────────────────────────
+ * Cream-on-ink. Activated by setting `data-theme="light"`
+ * on <html>; back-compat aliases (--background-color,
+ * --text-color, etc.) re-resolve through the cascade since
+ * they reference --bg/--matrix/--violet/etc. via var().
+ *
+ * Glows are removed (light mode = hard 1px borders, not
+ * neon haloes) and the scangrid line goes ink-on-cream. */
+html[data-theme="light"] {
+  --bg:              #dbdad6;
+  --matrix:          #047857;
+  --violet:          #6b21a8;
+  --panel:           #c9c7c2;
+  --border:          #1a1a1a;
+  --alert:           #b91c1c;
+
+  --fg-1: var(--matrix);
+  --fg-2: rgba(4, 120, 87, 0.80);
+  --fg-3: rgba(4, 120, 87, 0.60);
+  --fg-4: rgba(4, 120, 87, 0.40);
+
+  --matrix-tint-5:   rgba(4, 120, 87, 0.05);
+  --matrix-tint-10:  rgba(4, 120, 87, 0.10);
+  --matrix-tint-30:  rgba(4, 120, 87, 0.25);
+  --violet-tint-10:  rgba(107, 33, 168, 0.10);
+  --alert-tint-10:   rgba(185, 28, 28, 0.10);
+
+  /* Light mode trades neon glow for hard borders — keep the
+   * vars defined so .btn:hover etc. don't break, just no-op them. */
+  --matrix-glow:       none;
+  --matrix-green-glow: none;
+  --violet-glow:       none;
+  --matrix-glow-lg:    none;
+  --shadow-panel:      0 1px 0 rgba(0, 0, 0, 0.08);
+
+  --grid-line: rgba(0, 0, 0, 0.06);
+}
+
+html[data-theme="light"][data-accent="violet"] {
+  --accent:         var(--violet);
+  --accent-tint-10: var(--violet-tint-10);
+  --accent-tint-30: rgba(107, 33, 168, 0.25);
+  --accent-glow:    none;
+}
+
+html[data-theme="light"]:not([data-accent="violet"]) {
+  --accent:         var(--matrix);
+  --accent-tint-10: var(--matrix-tint-10);
+  --accent-tint-30: var(--matrix-tint-30);
+  --accent-glow:    none;
+}
+
 *, *::before, *::after {
   box-sizing: border-box;
   margin: 0;