diff --git a/tests/mjs/29-background-patterns.test.mjs b/tests/mjs/29-background-patterns.test.mjs
new file mode 100644
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+++ b/tests/mjs/29-background-patterns.test.mjs
@@ -0,0 +1,216 @@
+/**
+ * Background Pattern Tests
+ *
+ * Validates that background patterns are correctly applied in both light and dark themes.
+ * Tests cover:
+ * - Pattern existence (not "none")
+ * - Correct background colors
+ * - Pattern application to body element
+ * - Theme switching maintains patterns
+ */
+
+import { test, expect } from '@playwright/test';
+
+const URL = process.env.TEST_URL || 'http://localhost:1999';
+const CURRENT_YEAR = new Date().getFullYear();
+
+test.describe('Background Patterns', () => {
+
+  test('Light theme has concentric squares pattern on light gray background', async ({ page }) => {
+    // Navigate to light theme
+    await page.goto(`${URL}/?lang=en`);
+    await page.evaluate(() => {
+      localStorage.setItem('color-theme-mode', 'light');
+      document.documentElement.setAttribute('data-color-theme', 'light');
+    });
+    await page.reload();
+    await page.waitForTimeout(500);
+
+    // Get computed styles
+    const styles = await page.evaluate(() => {
+      const body = document.body;
+      const computed = window.getComputedStyle(body);
+      return {
+        backgroundColor: computed.backgroundColor,
+        backgroundImage: computed.backgroundImage,
+        backgroundSize: computed.backgroundSize,
+        backgroundAttachment: computed.backgroundAttachment,
+        themeAttr: document.documentElement.getAttribute('data-color-theme')
+      };
+    });
+
+    // Verify background color is light gray (#d6d6d6)
+    expect(styles.backgroundColor).toBe('rgb(214, 214, 214)');
+
+    // Verify pattern exists (not "none")
+    expect(styles.backgroundImage).not.toBe('none');
+
+    // Verify it's a concentric squares pattern (contains repeating-linear-gradient)
+    expect(styles.backgroundImage).toContain('repeating-linear-gradient');
+
+    // Verify background size is set (may have multiple values for multiple gradient layers)
+    expect(styles.backgroundSize).toContain('40px 40px');
+
+    // Verify background attachment is fixed (may have multiple values for multiple gradient layers)
+    expect(styles.backgroundAttachment).toContain('fixed');
+
+    console.log('✅ Light theme: Light gray background (#d6d6d6) with concentric squares pattern');
+  });
+
+  test('Dark theme has diagonal green grid pattern on medium gray background', async ({ page }) => {
+    // Navigate to dark theme
+    await page.goto(`${URL}/?lang=en`);
+    await page.evaluate(() => {
+      localStorage.setItem('color-theme-mode', 'dark');
+      document.documentElement.setAttribute('data-color-theme', 'dark');
+    });
+    await page.reload();
+    await page.waitForTimeout(500);
+
+    // Get computed styles
+    const styles = await page.evaluate(() => {
+      const body = document.body;
+      const computed = window.getComputedStyle(body);
+      return {
+        backgroundColor: computed.backgroundColor,
+        backgroundImage: computed.backgroundImage,
+        backgroundSize: computed.backgroundSize,
+        backgroundAttachment: computed.backgroundAttachment,
+        themeAttr: document.documentElement.getAttribute('data-color-theme')
+      };
+    });
+
+    // Verify background color is medium gray (#3a3a3a)
+    expect(styles.backgroundColor).toBe('rgb(58, 58, 58)');
+
+    // Verify pattern exists (not "none")
+    expect(styles.backgroundImage).not.toBe('none');
+
+    // Verify it's a diagonal grid pattern (contains 45deg and green color)
+    expect(styles.backgroundImage).toContain('45deg');
+    expect(styles.backgroundImage).toContain('rgba(0, 255, 128');
+
+    // Verify background size is set (may have multiple values for multiple gradient layers)
+    expect(styles.backgroundSize).toContain('40px 40px');
+
+    // Verify background attachment is fixed (may have multiple values for multiple gradient layers)
+    expect(styles.backgroundAttachment).toContain('fixed');
+
+    console.log('✅ Dark theme: Medium gray background (#3a3a3a) with diagonal green grid pattern');
+  });
+
+  test('Pattern persists when switching between themes', async ({ page }) => {
+    await page.goto(`${URL}/?lang=en`);
+
+    // Start with light theme
+    await page.evaluate(() => {
+      localStorage.setItem('color-theme-mode', 'light');
+      document.documentElement.setAttribute('data-color-theme', 'light');
+    });
+    await page.reload();
+    await page.waitForTimeout(500);
+
+    const lightPattern = await page.evaluate(() => {
+      return window.getComputedStyle(document.body).backgroundImage;
+    });
+
+    expect(lightPattern).not.toBe('none');
+    expect(lightPattern).toContain('repeating-linear-gradient');
+
+    // Switch to dark theme
+    await page.evaluate(() => {
+      localStorage.setItem('color-theme-mode', 'dark');
+      document.documentElement.setAttribute('data-color-theme', 'dark');
+    });
+    await page.reload();
+    await page.waitForTimeout(500);
+
+    const darkPattern = await page.evaluate(() => {
+      return window.getComputedStyle(document.body).backgroundImage;
+    });
+
+    expect(darkPattern).not.toBe('none');
+    expect(darkPattern).toContain('45deg');
+    expect(darkPattern).toContain('rgba(0, 255, 128');
+
+    // Patterns should be different
+    expect(lightPattern).not.toBe(darkPattern);
+
+    console.log('✅ Patterns persist and change correctly when switching themes');
+  });
+
+  test('Auto theme uses correct pattern based on system preference', async ({ page }) => {
+    // Test auto theme in light mode
+    await page.emulateMedia({ colorScheme: 'light' });
+    await page.goto(`${URL}/?lang=en`);
+    await page.evaluate(() => {
+      localStorage.setItem('color-theme-mode', 'auto');
+      document.documentElement.setAttribute('data-color-theme', 'auto');
+    });
+    await page.reload();
+    await page.waitForTimeout(500);
+
+    const autoLightStyles = await page.evaluate(() => {
+      const computed = window.getComputedStyle(document.body);
+      return {
+        backgroundColor: computed.backgroundColor,
+        backgroundImage: computed.backgroundImage
+      };
+    });
+
+    // In light mode, auto theme should use light gray background with concentric squares pattern
+    expect(autoLightStyles.backgroundColor).toBe('rgb(214, 214, 214)');
+    expect(autoLightStyles.backgroundImage).toContain('repeating-linear-gradient');
+
+    // Test auto theme in dark mode
+    await page.emulateMedia({ colorScheme: 'dark' });
+    await page.reload();
+    await page.waitForTimeout(500);
+
+    const autoDarkStyles = await page.evaluate(() => {
+      const computed = window.getComputedStyle(document.body);
+      return {
+        backgroundColor: computed.backgroundColor,
+        backgroundImage: computed.backgroundImage
+      };
+    });
+
+    // In dark mode, auto theme should use medium gray background with diagonal grid
+    expect(autoDarkStyles.backgroundColor).toBe('rgb(58, 58, 58)');
+    expect(autoDarkStyles.backgroundImage).toContain('45deg');
+    expect(autoDarkStyles.backgroundImage).toContain('rgba(0, 255, 128');
+
+    console.log('✅ Auto theme uses correct patterns based on system preference');
+  });
+
+  test('Take visual screenshots of both patterns for manual verification', async ({ page }) => {
+    // Light mode screenshot
+    await page.goto(`${URL}/?lang=en`);
+    await page.evaluate(() => {
+      localStorage.setItem('color-theme-mode', 'light');
+      document.documentElement.setAttribute('data-color-theme', 'light');
+    });
+    await page.reload();
+    await page.waitForTimeout(500);
+
+    await page.screenshot({
+      path: 'tests/screenshots/pattern-light-theme.png',
+      fullPage: false
+    });
+
+    // Dark mode screenshot
+    await page.evaluate(() => {
+      localStorage.setItem('color-theme-mode', 'dark');
+      document.documentElement.setAttribute('data-color-theme', 'dark');
+    });
+    await page.reload();
+    await page.waitForTimeout(500);
+
+    await page.screenshot({
+      path: 'tests/screenshots/pattern-dark-theme.png',
+      fullPage: false
+    });
+
+    console.log('✅ Screenshots saved to tests/screenshots/');
+  });
+});