diff --git a/.gitignore b/.gitignore
index bf945d3..a252c68 100644
--- a/.gitignore
+++ b/.gitignore
@@ -22,3 +22,6 @@
 npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
+
+# internal
+ROADMAP.md
diff --git a/src/index.test.ts b/src/index.test.ts
index 7c4a852..6857863 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -99,4 +99,27 @@ describe("toTypes", () => {
     expect(toTypes()).toContain("light");
     expect(toTypes()).toContain("dark");
   });
+
+  it("returns empty string when themes is empty", () => {
+    const { toTypes: toTypesEmpty } = defineThemes({
+      prefix: "ui",
+      tokens: ["accent"] as const,
+      themes: {},
+    });
+    expect(toTypesEmpty()).toBe("");
+  });
+});
+
+describe("inject — edge cases", () => {
+  it("does nothing when document.head is null", () => {
+    const { inject } = defineThemes({
+      prefix: "edge",
+      tokens: ["accent"] as const,
+      themes: { light: ["#fff"] },
+    });
+    const original = document.head;
+    Object.defineProperty(document, "head", { value: null, configurable: true });
+    expect(() => inject()).not.toThrow();
+    Object.defineProperty(document, "head", { value: original, configurable: true });
+  });
 });
diff --git a/src/index.ts b/src/index.ts
index 86a48e8..cbf63ca 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -10,7 +10,7 @@ export const defineThemes = ({
 }: DefineThemesConfig) => {
   const map: Record<string, CSSVarItem<typeof prefix, typeof tokens>> = {};
 
-  for (const t in themes) {
+  for (const t of Object.keys(themes)) {
     const vars: Record<string, string> = {};
     tokens.forEach((token, i) => {
       vars[key(token, prefix)] = themes[t]?.[i] ?? "";
@@ -23,7 +23,7 @@ export const defineThemes = ({
   const toCSS = () => {
     let r = "",
       first = true;
-    for (const t in map) {
+    for (const t of Object.keys(map)) {
       if (first) {
         r += `:root,\n`;
         first = false;
@@ -38,7 +38,7 @@ export const defineThemes = ({
   };
 
   const inject = () => {
-    if (typeof document === "undefined") return;
+    if (typeof document === "undefined" || !document.head) return;
     const id = `${VARTH}-${prefix}`;
     const el =
       document.getElementById(id) ??
@@ -51,11 +51,13 @@ export const defineThemes = ({
   const themeNames = Object.keys(map);
 
   const toTypes = () => {
+    if (themeNames.length === 0) return "";
+    const firstTheme = themeNames[0] as string;
     const union = (names: string[]) =>
       names.map((n) => `  | '${n}'`).join("\n");
     return [
       `export type ${THEME_TOKEN} =`,
-      union(Object.keys(getVarths(themeNames[0] ?? ""))),
+      union(Object.keys(getVarths(firstTheme))),
       ``,
       `export type ${THEME_NAME} =`,
       union(themeNames),
diff --git a/src/react/index.test.tsx b/src/react/index.test.tsx
index 6f03908..0e4b60b 100644
--- a/src/react/index.test.tsx
+++ b/src/react/index.test.tsx
@@ -5,6 +5,7 @@ import { afterEach, describe, expect, it } from "vitest";
 
 afterEach(() => {
   cleanup();
+  localStorage.clear();
 });
 
 const { inject, themeNames } = defineThemes({
@@ -70,4 +71,34 @@ describe("useTheme", () => {
     fireEvent.click(screen.getByTestId("button-click"));
     expect(screen.getByText("current: dark")).not.toBeNull();
   });
+
+  it("setTheme ignores invalid theme names", () => {
+    const InvalidSwitcher = () => {
+      const { theme, setTheme } = useTheme();
+      return (
+        <button data-testid="invalid" onClick={() => setTheme("nonexistent")}>
+          current: {theme}
+        </button>
+      );
+    };
+    render(
+      <ThemeProvider default="light" inject={inject} themeNames={themeNames}>
+        <InvalidSwitcher />
+      </ThemeProvider>,
+    );
+    fireEvent.click(screen.getByTestId("invalid"));
+    expect(screen.getByText("current: light")).not.toBeNull();
+  });
+
+  it("setTheme does not throw when localStorage.setItem throws", () => {
+    const original = localStorage.setItem.bind(localStorage);
+    localStorage.setItem = () => { throw new Error("QuotaExceededError"); };
+    render(
+      <ThemeProvider default="light" inject={inject} themeNames={themeNames}>
+        <ThemeSwitcher />
+      </ThemeProvider>,
+    );
+    expect(() => fireEvent.click(screen.getByTestId("button-click"))).not.toThrow();
+    localStorage.setItem = original;
+  });
 });
diff --git a/src/react/index.tsx b/src/react/index.tsx
index d3c3bd2..a297384 100644
--- a/src/react/index.tsx
+++ b/src/react/index.tsx
@@ -35,13 +35,20 @@ export const ThemeProvider = ({
   );
 
   const setTheme = (next: string) => {
+    if (!themeNames.includes(next)) return;
     setThemeState(next);
-    localStorage.setItem("varth-theme", next);
+    if (typeof localStorage !== "undefined") {
+      try {
+        localStorage.setItem("varth-theme", next);
+      } catch {
+        // ignore QuotaExceededError / SecurityError
+      }
+    }
   };
 
   useEffect(() => {
     inject();
-  }, []);
+  }, [inject]);
 
   return (
     <Ctx.Provider value={{ theme, setTheme, themeNames }}>