diff --git a/IMPLEMENTATION_PLAN.md b/IMPLEMENTATION_PLAN.md
index c66ee53..6073121 100644
--- a/IMPLEMENTATION_PLAN.md
+++ b/IMPLEMENTATION_PLAN.md
@@ -4,7 +4,7 @@ This file is maintained by Ralph. Run `./ralph-sandbox.sh plan 3` to generate ta
 
 ## Current Status: Feature Complete
 
-**Test Coverage:** 1014 unit tests (51 files) + 202 E2E tests (13 files) = 1216 total tests
+**Test Coverage:** 1014 unit tests (51 files) + 204 E2E tests (14 files) = 1218 total tests
 
 All P0-P5 items are complete. The project is feature complete.
 
@@ -97,7 +97,7 @@ All P0-P5 items are complete. The project is feature complete.
 | PeriodDateModal | 22 | Period input modal |
 | Skeletons | 29 | Loading states with shimmer |
 
-### E2E Tests (13 files, 202 tests)
+### E2E Tests (14 files, 204 tests)
 | File | Tests | Coverage |
 |------|-------|----------|
 | smoke.spec.ts | 3 | Basic app functionality |
@@ -113,6 +113,7 @@ All P0-P5 items are complete. The project is feature complete.
 | plan.spec.ts | 7 | Plan page |
 | health.spec.ts | 3 | Health/observability |
 | mobile.spec.ts | 7 | Mobile viewport behavior, responsive layout, calendar mobile |
+| dark-mode.spec.ts | 2 | System preference detection, light/dark theme application |
 
 ---
 
@@ -124,11 +125,11 @@ These are optional enhancements to improve E2E coverage. Not required for featur
 | File | Tests | Description |
 |------|-------|-------------|
 | notifications.spec.ts | 3 | Notification preferences |
-| dark-mode.spec.ts | 2 | System preference detection |
 
 ### Completed Enhancements
 | File | Tests Added | Focus Area |
 |------|-------------|------------|
+| dark-mode.spec.ts | +2 | System preference detection (light/dark mode) |
 | garmin.spec.ts | +4 | Network error recovery (save, disconnect, status fetch, retry) |
 | calendar.spec.ts | +4 | Accessibility (ARIA, keyboard nav) |
 | settings.spec.ts | +1 | Error recovery on failed save |
@@ -149,6 +150,7 @@ These are optional enhancements to improve E2E coverage. Not required for featur
 
 ## Revision History
 
+- 2026-01-13: Added dark-mode.spec.ts with 2 E2E tests (system preference detection for light/dark mode)
 - 2026-01-13: Added 4 Garmin E2E tests (network error recovery for save, disconnect, status fetch, retry)
 - 2026-01-13: Added 8 E2E tests (calendar accessibility, settings error recovery, calendar mobile behavior)
 - 2026-01-13: Added mobile.spec.ts with 4 E2E tests (mobile viewport behavior, responsive layout)
diff --git a/e2e/dark-mode.spec.ts b/e2e/dark-mode.spec.ts
new file mode 100644
index 0000000..b0147ad
--- /dev/null
+++ b/e2e/dark-mode.spec.ts
@@ -0,0 +1,90 @@
+// ABOUTME: E2E tests for dark mode system preference detection.
+// ABOUTME: Verifies app respects OS-level dark mode setting via prefers-color-scheme.
+import { expect, test } from "@playwright/test";
+
+// Helper to parse background color and determine if it's light or dark
+function isLightBackground(colorValue: string): boolean {
+  // Try rgb/rgba format: rgb(255, 255, 255) or rgba(255, 255, 255, 1)
+  const rgbMatch = colorValue.match(
+    /rgba?\s*\(\s*([\d.]+)\s*,\s*([\d.]+)\s*,\s*([\d.]+)/,
+  );
+
+  if (rgbMatch) {
+    const r = parseFloat(rgbMatch[1]);
+    const g = parseFloat(rgbMatch[2]);
+    const b = parseFloat(rgbMatch[3]);
+    // Calculate relative luminance (simplified)
+    const luminance = (r + g + b) / 3;
+    return luminance > 128;
+  }
+
+  // Try oklch format: oklch(1 0 0) where first value is lightness
+  const oklchMatch = colorValue.match(/oklch\s*\(\s*([\d.]+)/);
+  if (oklchMatch) {
+    const lightness = parseFloat(oklchMatch[1]);
+    return lightness > 0.5;
+  }
+
+  // Try color() format with oklch: color(oklch 1 0 0)
+  const colorOklchMatch = colorValue.match(/color\s*\(\s*oklch\s+([\d.]+)/);
+  if (colorOklchMatch) {
+    const lightness = parseFloat(colorOklchMatch[1]);
+    return lightness > 0.5;
+  }
+
+  // Try lab() format: lab(100 0 0) where first value is lightness 0-100
+  const labMatch = colorValue.match(/lab\s*\(\s*([\d.]+)/);
+  if (labMatch) {
+    const lightness = parseFloat(labMatch[1]);
+    return lightness > 50;
+  }
+
+  // Default: couldn't parse, assume we need to fail the test
+  return false;
+}
+
+test.describe("dark mode", () => {
+  test("applies light mode styling when system prefers light", async ({
+    page,
+  }) => {
+    // Set system preference to light mode
+    await page.emulateMedia({ colorScheme: "light" });
+
+    await page.goto("/login");
+    await page.waitForLoadState("domcontentloaded");
+
+    // Get computed background color of the body
+    const bodyBgColor = await page.evaluate(() => {
+      return window.getComputedStyle(document.body).backgroundColor;
+    });
+
+    // In light mode, background should be white/light (oklch(1 0 0) = white)
+    const isLight = isLightBackground(bodyBgColor);
+    expect(
+      isLight,
+      `Expected light background in light mode, got: ${bodyBgColor}`,
+    ).toBe(true);
+  });
+
+  test("applies dark mode styling when system prefers dark", async ({
+    page,
+  }) => {
+    // Set system preference to dark mode
+    await page.emulateMedia({ colorScheme: "dark" });
+
+    await page.goto("/login");
+    await page.waitForLoadState("domcontentloaded");
+
+    // Get computed background color of the body
+    const bodyBgColor = await page.evaluate(() => {
+      return window.getComputedStyle(document.body).backgroundColor;
+    });
+
+    // In dark mode, background should be dark (oklch(0.145 0 0) = near black)
+    const isLight = isLightBackground(bodyBgColor);
+    expect(
+      isLight,
+      `Expected dark background in dark mode, got: ${bodyBgColor}`,
+    ).toBe(false);
+  });
+});