diff --git a/test/themes.test.ts b/test/themes.test.ts
new file mode 100644
index 000000000..f43750bdb
--- /dev/null
+++ b/test/themes.test.ts
@@ -0,0 +1,87 @@
+import { describe, expect, it } from "vitest";
+import {
+  isThemeId,
+  getThemeDefinition,
+  isDarkTheme,
+  nextThemeId,
+  THEME_OPTIONS,
+  DEFAULT_THEME,
+  type ThemeId,
+} from "../src/lib/themes";
+
+describe("isThemeId", () => {
+  it("returns true for all valid theme IDs", () => {
+    expect(isThemeId("classic-dark")).toBe(true);
+    expect(isThemeId("modern-light-blue")).toBe(true);
+    expect(isThemeId("nordic-frost")).toBe(true);
+    expect(isThemeId("cyberpunk-matrix")).toBe(true);
+  });
+
+  it("returns false for invalid strings", () => {
+    expect(isThemeId("random-theme")).toBe(false);
+    expect(isThemeId("dark")).toBe(false);
+    expect(isThemeId("CLASSIC-DARK")).toBe(false);
+    expect(isThemeId("")).toBe(false);
+  });
+
+  it("returns false for null and undefined", () => {
+    expect(isThemeId(null)).toBe(false);
+    expect(isThemeId(undefined)).toBe(false);
+  });
+});
+
+describe("getThemeDefinition", () => {
+  it("returns the correct definition for each valid theme ID", () => {
+    for (const theme of THEME_OPTIONS) {
+      const def = getThemeDefinition(theme.id);
+      expect(def.id).toBe(theme.id);
+      expect(def.name).toBe(theme.name);
+    }
+  });
+
+  it("returns the default theme for invalid ID", () => {
+    const def = getThemeDefinition("invalid" as ThemeId);
+    expect(def.id).toBe(DEFAULT_THEME);
+  });
+});
+
+describe("isDarkTheme", () => {
+  it("returns true for dark mode themes", () => {
+    expect(isDarkTheme("classic-dark")).toBe(true);
+    expect(isDarkTheme("nordic-frost")).toBe(true);
+    expect(isDarkTheme("cyberpunk-matrix")).toBe(true);
+  });
+
+  it("returns false for light mode themes", () => {
+    expect(isDarkTheme("modern-light-blue")).toBe(false);
+  });
+});
+
+describe("nextThemeId", () => {
+  it("cycles through all themes in order", () => {
+    const order = THEME_OPTIONS.map((t) => t.id);
+    for (let i = 0; i < order.length; i++) {
+      const current = order[i];
+      const next = nextThemeId(current);
+      const expectedNext = order[(i + 1) % order.length];
+      expect(next).toBe(expectedNext);
+    }
+  });
+
+  it("wraps from last theme back to first", () => {
+    const lastTheme = THEME_OPTIONS[THEME_OPTIONS.length - 1].id;
+    const firstTheme = THEME_OPTIONS[0].id;
+    expect(nextThemeId(lastTheme)).toBe(firstTheme);
+  });
+
+  it("falls back to first theme for invalid currentTheme", () => {
+    const firstTheme = THEME_OPTIONS[0].id;
+    expect(nextThemeId("unknown" as ThemeId)).toBe(firstTheme);
+  });
+
+  it("never returns the same theme consecutively", () => {
+    for (const theme of THEME_OPTIONS) {
+      expect(nextThemeId(theme.id)).not.toBe(theme.id);
+    }
+  });
+});