diff --git a/.gitignore b/.gitignore
index 438bcfe..867c58a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,8 @@ coverage
 .vercel
 .DS_Store
 .env*
+
+# IDD tree-lock (idd-tree-lock.sh #183) — per-machine session state, not tracked
+.claude/.idd/tree-lock
+.claude/.idd/state/
+.claude/.idd/attachments/
diff --git a/README.md b/README.md
index 983d50f..7c67b88 100644
--- a/README.md
+++ b/README.md
@@ -18,6 +18,8 @@
 
 做完按「複製測試結果」，JSON 會複製到剪貼簿（並下載備份），可貼給 AI 或維修人員判讀。
 
+介面深淺主題自動跟隨系統外觀（`prefers-color-scheme`），深色為預設設計、淺色完整支援。
+
 ### 判讀基準
 
 - 事件頻率：健康值約 90–120 Hz（120 為硬體上限）
@@ -50,6 +52,8 @@ Five interactive checks covering the five failure modes of a MacBook trackpad. T
 
 When done, press "Copy results": a JSON report is copied to the clipboard (and downloaded) — paste it to an AI or a technician.
 
+The UI follows your system appearance automatically (`prefers-color-scheme`) — dark is the primary design, light fully supported.
+
 ### Interpretation baseline
 
 - Event rate: ~90–120 Hz is healthy (120 is the hardware cap)
diff --git a/package-lock.json b/package-lock.json
index 5dec952..850b241 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -8,6 +8,7 @@
       "name": "mac-trackpad-test",
       "version": "1.0.0",
       "devDependencies": {
+        "@types/node": "^26.1.1",
         "@vitest/coverage-v8": "^4.1.10",
         "jsdom": "^29.1.1",
         "typescript": "^7.0.2",
@@ -713,6 +714,16 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@types/node": {
+      "version": "26.1.1",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-26.1.1.tgz",
+      "integrity": "sha512-nxAkRSVkN1Y0JC1W8ky/fTfkGsMmcrRsbx+3XoZE+rMOX71kLYTV7fLXpqud1GpbpP5TuffXFqfX7fH2GgZREw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "undici-types": "~8.3.0"
+      }
+    },
     "node_modules/@typescript/typescript-aix-ppc64": {
       "version": "7.0.2",
       "resolved": "https://registry.npmjs.org/@typescript/typescript-aix-ppc64/-/typescript-aix-ppc64-7.0.2.tgz",
@@ -2192,6 +2203,13 @@
         "node": ">=20.18.1"
       }
     },
+    "node_modules/undici-types": {
+      "version": "8.3.0",
+      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-8.3.0.tgz",
+      "integrity": "sha512-j375ScV60dom+YkPFIfTLcOiPxkN/buHz5GobjLhixFuANaNs3C9l4GmrWqejgXWJ7BbJcFYpTEUkS1Ge8bpZQ==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/vite": {
       "version": "8.1.4",
       "resolved": "https://registry.npmjs.org/vite/-/vite-8.1.4.tgz",
diff --git a/package.json b/package.json
index 80b44eb..6c5da73 100644
--- a/package.json
+++ b/package.json
@@ -12,6 +12,7 @@
     "coverage": "vitest run --coverage"
   },
   "devDependencies": {
+    "@types/node": "^26.1.1",
     "@vitest/coverage-v8": "^4.1.10",
     "jsdom": "^29.1.1",
     "typescript": "^7.0.2",
diff --git a/src/panels/trail.ts b/src/panels/trail.ts
index 054d269..63fa6c2 100644
--- a/src/panels/trail.ts
+++ b/src/panels/trail.ts
@@ -1,6 +1,7 @@
 import { detectStep, type Point } from '../core/jump-detector';
 import type { Session } from '../core/session';
 import { t } from '../i18n';
+import { cssColor } from '../theme';
 
 export function mountTrail(el: HTMLElement, session: Session): void {
   el.innerHTML = `
@@ -16,6 +17,9 @@ export function mountTrail(el: HTMLElement, session: Session): void {
 
   const canvas = el.querySelector('canvas') as HTMLCanvasElement;
   const ctx2d = canvas.getContext('2d'); // jsdom 回 null，繪圖跳過、邏輯照常
+  // mount 時解析一次主題色；jsdom / 舊瀏覽器退 fallback 固定色
+  const colorStroke = cssColor('--blue', '#0a84ff');
+  const colorJump = cssColor('--red', '#ff453a');
   const hzEl = el.querySelector('.hz') as HTMLElement;
   const maxJumpEl = el.querySelector('.maxjump') as HTMLElement;
   const jumpsEl = el.querySelector('.jumps') as HTMLElement;
@@ -60,7 +64,7 @@ export function mountTrail(el: HTMLElement, session: Session): void {
       jumpsEl.textContent = String(session.trail.jumps.length);
     }
     if (ctx2d && last) {
-      ctx2d.strokeStyle = step.isJump ? '#ff453a' : '#0a84ff';
+      ctx2d.strokeStyle = step.isJump ? colorJump : colorStroke;
       ctx2d.beginPath();
       ctx2d.moveTo(last.x, last.y);
       ctx2d.lineTo(curr.x, curr.y);
diff --git a/src/styles.css b/src/styles.css
index 5093dde..672658c 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -1,58 +1,106 @@
-:root { color-scheme: dark; }
+:root {
+  color-scheme: light dark;
+  /* 深色（預設）色板 — iOS 系統色系 */
+  --bg: #1c1c1e;
+  --card: #2c2c2e;
+  --inset: #1c1c1e;
+  --text: #f2f2f7;
+  --text-dim: #98989f;
+  --blue: #0a84ff;
+  --green: #30d158;
+  --red: #ff453a;
+  --yellow: #ffd60a;
+  --btn-bg: #48484a;
+  --btn-text: #f2f2f7;
+  --banner-bg: #3a2f00;
+  --banner-text: #ffd60a;
+  --cell-left-bg: #1f4d2e;
+  --cell-left-text: #7ee2a0;
+  --cell-both-bg: #1e3a5f;
+  --cell-both-text: #8ec9ff;
+  --on-accent: #f2f2f7;
+  --thresh: #f2f2f7aa;
+}
+
+@media (prefers-color-scheme: light) {
+  :root {
+    --bg: #f2f2f7;
+    --card: #ffffff;
+    --inset: #e5e5ea;
+    --text: #1c1c1e;
+    --text-dim: #5a5a5f;
+    --blue: #0071e3;
+    --green: #1d7a3d;
+    --red: #d70015;
+    --yellow: #b25000;
+    --btn-bg: #d1d1d6;
+    --btn-text: #1c1c1e;
+    --banner-bg: #fff4cc;
+    --banner-text: #7a5200;
+    --cell-left-bg: #d7f5e0;
+    --cell-left-text: #1d7a3d;
+    --cell-both-bg: #d6e9ff;
+    --cell-both-text: #1d5fa0;
+    --on-accent: #ffffff;
+    --thresh: #1c1c1eaa;
+  }
+}
+
 * { box-sizing: border-box; margin: 0; padding: 0; }
 body {
   font-family: -apple-system, "PingFang TC", sans-serif;
-  background: #1c1c1e; color: #f2f2f7;
+  background: var(--bg); color: var(--text);
   padding: 24px; max-width: 900px; margin: 0 auto;
 }
 header { display: flex; justify-content: space-between; align-items: flex-start; gap: 12px; margin-bottom: 20px; }
 h1 { font-size: 22px; margin-bottom: 4px; }
-.sub { color: #98989f; font-size: 13px; }
+.sub { color: var(--text-dim); font-size: 13px; }
 .lang-btn {
-  background: #48484a; color: #f2f2f7; border: none; border-radius: 6px;
+  background: var(--btn-bg); color: var(--btn-text); border: none; border-radius: 6px;
   padding: 6px 14px; font-size: 13px; cursor: pointer; flex-shrink: 0;
 }
 .banner {
-  background: #3a2f00; color: #ffd60a; border-radius: 10px;
+  background: var(--banner-bg); color: var(--banner-text); border-radius: 10px;
   padding: 12px 16px; font-size: 13px; margin-bottom: 16px;
 }
-section { background: #2c2c2e; border-radius: 12px; padding: 16px; margin-bottom: 16px; }
+section { background: var(--card); border-radius: 12px; padding: 16px; margin-bottom: 16px; }
 h2 { font-size: 15px; margin-bottom: 4px; }
-.hint { color: #98989f; font-size: 12px; margin-bottom: 10px; }
+.hint { color: var(--text-dim); font-size: 12px; margin-bottom: 10px; }
 .notice-card {
-  background: #1c1c1e; border-radius: 8px; padding: 14px;
-  font-size: 13px; color: #98989f; margin-top: 8px;
+  background: var(--inset); border-radius: 8px; padding: 14px;
+  font-size: 13px; color: var(--text-dim); margin-top: 8px;
   display: flex; flex-direction: column; gap: 4px;
 }
-.notice-card b { color: #ffd60a; }
-.trail-canvas { background: #1c1c1e; border-radius: 8px; display: block; width: 100%; touch-action: none; }
+.notice-card b { color: var(--banner-text); }
+.trail-canvas { background: var(--inset); border-radius: 8px; display: block; width: 100%; touch-action: none; }
 button {
-  background: #48484a; color: #f2f2f7; border: none; border-radius: 6px;
+  background: var(--btn-bg); color: var(--btn-text); border: none; border-radius: 6px;
   padding: 4px 12px; font-size: 12px; cursor: pointer; margin-top: 8px;
 }
 .grid3 { display: grid; grid-template-columns: repeat(3, 1fr); gap: 6px; height: 220px; }
 .cell {
-  background: #1c1c1e; border-radius: 8px; display: flex;
+  background: var(--inset); border-radius: 8px; display: flex;
   align-items: center; justify-content: center;
-  font-size: 12px; color: #98989f; user-select: none; -webkit-user-select: none;
+  font-size: 12px; color: var(--text-dim); user-select: none; -webkit-user-select: none;
 }
-.cell.leftdone { background: #1f4d2e; color: #7ee2a0; }
-.cell.bothdone { background: #1e3a5f; color: #8ec9ff; }
+.cell.leftdone { background: var(--cell-left-bg); color: var(--cell-left-text); }
+.cell.bothdone { background: var(--cell-both-bg); color: var(--cell-both-text); }
 .stats { display: flex; gap: 18px; flex-wrap: wrap; margin-top: 10px; font-size: 13px; }
 .stats b { font-variant-numeric: tabular-nums; }
-.ok { color: #30d158; }
-.forcebar-wrap { background: #1c1c1e; border-radius: 8px; height: 26px; overflow: hidden; position: relative; }
+.ok { color: var(--green); }
+.forcebar-wrap { background: var(--inset); border-radius: 8px; height: 26px; overflow: hidden; position: relative; }
 .forcebar {
-  background: linear-gradient(90deg, #30d158, #ffd60a, #ff453a);
+  background: linear-gradient(90deg, var(--green), var(--yellow), var(--red));
   height: 100%; width: 100%; transform: scaleX(0); transform-origin: left;
 }
-.force-thresh { position: absolute; top: 0; bottom: 0; left: 33%; width: 2px; background: #f2f2f7aa; }
+.force-thresh { position: absolute; top: 0; bottom: 0; left: 33%; width: 2px; background: var(--thresh); }
 .scrollbox {
-  height: 140px; overflow: scroll; background: #1c1c1e; border-radius: 8px;
-  padding: 10px; font-size: 12px; color: #98989f; line-height: 2.2;
+  height: 140px; overflow: scroll; background: var(--inset); border-radius: 8px;
+  padding: 10px; font-size: 12px; color: var(--text-dim); line-height: 2.2;
 }
-.pinchbox { height: 120px; background: #1c1c1e; border-radius: 8px; display: flex; align-items: center; justify-content: center; }
+.pinchbox { height: 120px; background: var(--inset); border-radius: 8px; display: flex; align-items: center; justify-content: center; }
 .pinchtarget {
-  width: 60px; height: 60px; background: #0a84ff; border-radius: 12px;
+  width: 60px; height: 60px; background: var(--blue); border-radius: 12px;
   display: flex; align-items: center; justify-content: center; font-size: 11px;
+  color: var(--on-accent);
 }
diff --git a/src/theme.ts b/src/theme.ts
new file mode 100644
index 0000000..d4139e3
--- /dev/null
+++ b/src/theme.ts
@@ -0,0 +1,12 @@
+// 讀取 CSS 自訂屬性。canvas 繪圖不吃 CSS 變數，需在 JS 端解析。
+// jsdom 的 computed style 可讀 inline 自訂屬性，但測試環境不載入 styles.css，
+// 變數解析為空字串 → 退 fallback；SSR / 極舊環境無 getComputedStyle 同樣退 fallback。
+export function cssColor(varName: string, fallback: string): string {
+  if (typeof window === 'undefined' || typeof getComputedStyle !== 'function') {
+    return fallback;
+  }
+  const computed = getComputedStyle(document.documentElement)
+    .getPropertyValue(varName)
+    .trim();
+  return computed || fallback;
+}
diff --git a/tests/contrast.test.ts b/tests/contrast.test.ts
new file mode 100644
index 0000000..cf19ef6
--- /dev/null
+++ b/tests/contrast.test.ts
@@ -0,0 +1,88 @@
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+
+// WCAG 2.x relative luminance + contrast ratio（機械驗證，取代人工判讀）
+function srgbToLinear(c: number): number {
+  const v = c / 255;
+  return v <= 0.04045 ? v / 12.92 : ((v + 0.055) / 1.055) ** 2.4;
+}
+
+function luminance(hex: string): number {
+  const m = /^#([0-9a-f]{6})$/i.exec(hex.trim());
+  if (!m) throw new Error(`非 6 位 hex：${hex}`);
+  const n = parseInt(m[1], 16);
+  const r = srgbToLinear((n >> 16) & 0xff);
+  const g = srgbToLinear((n >> 8) & 0xff);
+  const b = srgbToLinear(n & 0xff);
+  return 0.2126 * r + 0.7152 * g + 0.0722 * b;
+}
+
+function contrast(fg: string, bg: string): number {
+  const l1 = luminance(fg);
+  const l2 = luminance(bg);
+  const [hi, lo] = l1 > l2 ? [l1, l2] : [l2, l1];
+  return (hi + 0.05) / (lo + 0.05);
+}
+
+// 從 styles.css 抽出兩套色板：:root 區塊（深色預設）與 @media light 覆寫
+function extractPalettes(): { dark: Record<string, string>; light: Record<string, string> } {
+  const css = readFileSync(resolve(process.cwd(), 'src/styles.css'), 'utf8');
+  const grab = (block: string): Record<string, string> => {
+    const vars: Record<string, string> = {};
+    for (const m of block.matchAll(/(--[a-z-]+):\s*(#[0-9a-fA-F]{6})\s*;/g)) {
+      vars[m[1]] = m[2];
+    }
+    return vars;
+  };
+  const lightStart = css.indexOf('@media (prefers-color-scheme: light)');
+  if (lightStart < 0) throw new Error('styles.css 缺 light 覆寫區塊');
+  const darkVars = grab(css.slice(0, lightStart));
+  const lightOverrides = grab(css.slice(lightStart, css.indexOf('}', css.indexOf('}', lightStart) + 1) + 1));
+  return { dark: darkVars, light: { ...darkVars, ...lightOverrides } };
+}
+
+// 文字/背景配對與門檻。資訊性文字 4.5（AA）。
+// --on-accent on --blue（pinch 目標的裝飾性標籤）採 per-theme 門檻：
+// - 深色 3.0：launch 基線（main）的 .pinchtarget 無 color 屬性、繼承 body #f2f2f7，
+//   對 --blue #0a84ff = 3.27 —— pre-existing，且 issue Expected(4) 深色不變優先。
+// - 淺色 4.5：R1 blocking 修正後鎖定，不允許回退（verify R2 finding）。
+type Threshold = number | { dark: number; light: number };
+const TEXT_PAIRS: Array<{ fg: string; bg: string; min: Threshold; note: string }> = [
+  { fg: '--text', bg: '--bg', min: 4.5, note: '本文 on 頁底' },
+  { fg: '--text', bg: '--card', min: 4.5, note: '本文 on 卡片' },
+  { fg: '--text-dim', bg: '--card', min: 4.5, note: 'hint on 卡片' },
+  { fg: '--text-dim', bg: '--inset', min: 4.5, note: 'cell/scrollbox/notice 文字 on inset' },
+  { fg: '--btn-text', bg: '--btn-bg', min: 4.5, note: '按鈕' },
+  { fg: '--banner-text', bg: '--banner-bg', min: 4.5, note: 'banner' },
+  { fg: '--cell-left-text', bg: '--cell-left-bg', min: 4.5, note: '九宮格左鍵完成' },
+  { fg: '--cell-both-text', bg: '--cell-both-bg', min: 4.5, note: '九宮格雙完成' },
+  { fg: '--green', bg: '--card', min: 4.5, note: '.ok 狀態文字' },
+  { fg: '--on-accent', bg: '--blue', min: { dark: 3.0, light: 4.5 }, note: 'pinch 目標裝飾標籤（per-theme 門檻，見上）' },
+];
+
+describe.each(['dark', 'light'] as const)('%s 色板 WCAG 對比', (theme) => {
+  const palettes = extractPalettes();
+  const palette = palettes[theme];
+
+  it.each(TEXT_PAIRS)('$fg on $bg ≥ $min（$note）', ({ fg, bg, min }) => {
+    expect(palette[fg], `${theme} 缺變數 ${fg}`).toBeDefined();
+    expect(palette[bg], `${theme} 缺變數 ${bg}`).toBeDefined();
+    const threshold = typeof min === 'number' ? min : min[theme];
+    const ratio = contrast(palette[fg], palette[bg]);
+    expect(ratio, `${theme}: ${fg}(${palette[fg]}) on ${bg}(${palette[bg]}) = ${ratio.toFixed(2)}`).toBeGreaterThanOrEqual(threshold);
+  });
+});
+
+describe('深色預設值回歸鎖（issue Expected(4)：既有深色外觀不變）', () => {
+  it('深色核心變數與上線版 hex 完全一致', () => {
+    const { dark } = extractPalettes();
+    expect(dark['--bg']).toBe('#1c1c1e');
+    expect(dark['--card']).toBe('#2c2c2e');
+    expect(dark['--text']).toBe('#f2f2f7');
+    expect(dark['--text-dim']).toBe('#98989f');
+    expect(dark['--blue']).toBe('#0a84ff');
+    expect(dark['--green']).toBe('#30d158');
+    expect(dark['--red']).toBe('#ff453a');
+    expect(dark['--yellow']).toBe('#ffd60a');
+  });
+});
diff --git a/tests/theme.test.ts b/tests/theme.test.ts
new file mode 100644
index 0000000..c4c3bee
--- /dev/null
+++ b/tests/theme.test.ts
@@ -0,0 +1,30 @@
+import { cssColor } from '../src/theme';
+
+describe('cssColor', () => {
+  afterEach(() => {
+    document.documentElement.style.removeProperty('--test-color');
+  });
+
+  it('讀取已定義的 CSS 變數', () => {
+    document.documentElement.style.setProperty('--test-color', '#123456');
+    expect(cssColor('--test-color', '#fallback')).toBe('#123456');
+  });
+
+  it('未定義時回 fallback', () => {
+    expect(cssColor('--never-defined', '#abcdef')).toBe('#abcdef');
+  });
+
+  it('無 getComputedStyle（SSR/極舊環境）時回 fallback', () => {
+    vi.stubGlobal('getComputedStyle', undefined);
+    try {
+      expect(cssColor('--test-color', '#fa11ba')).toBe('#fa11ba');
+    } finally {
+      vi.unstubAllGlobals();
+    }
+  });
+
+  it('值含空白時 trim', () => {
+    document.documentElement.style.setProperty('--test-color', '  #654321  ');
+    expect(cssColor('--test-color', '#fallback')).toBe('#654321');
+  });
+});
diff --git a/tsconfig.json b/tsconfig.json
index 5e6f50f..ac74002 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -13,7 +13,7 @@
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "noFallthroughCasesInSwitch": true,
-    "types": ["vitest/globals"]
+    "types": ["vitest/globals", "node"]
   },
   "include": ["src", "tests"]
 }