test(sanitizer): add theme color parsing fallback boundary tests (JhaSourav07#1652)

## Description
Adds a new `theme color parsing fallbacks` describe block to
`lib/svg/sanitizer.test.ts` that verifies the `hexColor` utility
correctly falls back to default theme colors when passed invalid hex
names.

Fixes JhaSourav07#1552

## Pillar
- [x] Other (Bug fix, refactoring, docs)

## Checklist before requesting a review:
- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors.
- [x] My commits follow the Conventional Commits format.
- [x] I have made sure that i have only one commit to merge in this PR.

Author: JhaSourav07

lib/svg/sanitizer.test.ts

@@ -236,4 +236,36 @@ describe('SVG Sanitizer Utilities', () => {
       expect(sanitizeGoogleFontUrl('https://example.com')).toBe(null);
     });
   });
+
+  describe('theme color parsing fallbacks', () => {
+    it('resolves invalid hex names to default dark theme background color', () => {
+      expect(hexColor('not-a-background-color', '0d1117')).toBe('0d1117');
+      expect(hexColor('background', '0d1117')).toBe('0d1117');
+      expect(hexColor('', '0d1117')).toBe('0d1117');
+    });
+
+    it('resolves invalid hex names to default dark theme text color', () => {
+      expect(hexColor('not-a-text-color', 'c9d1d9')).toBe('c9d1d9');
+      expect(hexColor('text', 'c9d1d9')).toBe('c9d1d9');
+      expect(hexColor('invalid-text', 'c9d1d9')).toBe('c9d1d9');
+    });
+
+    it('resolves invalid hex names to default dark theme accent color', () => {
+      expect(hexColor('not-an-accent-color', '58a6ff')).toBe('58a6ff');
+      expect(hexColor('accent', '58a6ff')).toBe('58a6ff');
+      expect(hexColor('highlight', '58a6ff')).toBe('58a6ff');
+    });
+
+    it('resolves invalid hex names to light theme colors', () => {
+      expect(hexColor('not-a-color', 'ffffff')).toBe('ffffff');
+      expect(hexColor('not-a-color', '24292f')).toBe('24292f');
+      expect(hexColor('not-a-color', '0969da')).toBe('0969da');
+    });
+
+    it('still resolves valid hex correctly even with theme-like fallbacks', () => {
+      expect(hexColor('0d1117', '000000')).toBe('0d1117');
+      expect(hexColor('c9d1d9', '000000')).toBe('c9d1d9');
+      expect(hexColor('58a6ff', '000000')).toBe('58a6ff');
+    });
+  });
 });