diff --git a/types/index.theme-contrast.test.ts b/types/index.theme-contrast.test.ts
new file mode 100644
index 000000000..12da5f341
--- /dev/null
+++ b/types/index.theme-contrast.test.ts
@@ -0,0 +1,48 @@
+import { describe, it, expect } from 'vitest';
+import { isLocDay } from './index';
+import type { ContributionDay } from './index';
+
+describe('types/index - Dark and Light Prefers-Color-Scheme Visual Cohesion', () => {
+  it('isLocDay returns false for a standard ContributionDay without LoC fields', () => {
+    const day: ContributionDay = { contributionCount: 5, date: '2025-06-01' };
+    expect(isLocDay(day)).toBe(false);
+  });
+
+  it('isLocDay returns true when both locAdditions and locDeletions are present as numbers', () => {
+    const day: ContributionDay = {
+      contributionCount: 5,
+      date: '2025-06-01',
+      locAdditions: 150,
+      locDeletions: 30,
+    };
+    expect(isLocDay(day)).toBe(true);
+  });
+
+  it('isLocDay returns false when locAdditions is undefined even if locDeletions is a number', () => {
+    const day: ContributionDay = {
+      contributionCount: 5,
+      date: '2025-06-01',
+      locDeletions: 30,
+    };
+    expect(isLocDay(day)).toBe(false);
+  });
+
+  it('isLocDay returns false when locDeletions is undefined even if locAdditions is a number', () => {
+    const day: ContributionDay = {
+      contributionCount: 5,
+      date: '2025-06-01',
+      locAdditions: 150,
+    };
+    expect(isLocDay(day)).toBe(false);
+  });
+
+  it('isLocDay returns true when loc fields are zero — zero is a valid number', () => {
+    const day: ContributionDay = {
+      contributionCount: 0,
+      date: '2025-06-01',
+      locAdditions: 0,
+      locDeletions: 0,
+    };
+    expect(isLocDay(day)).toBe(true);
+  });
+});