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React Theme Toggle with Persisted Light/Dark Mode

By FrontendAtlas Team · Updated Jan 30, 2026

Add a global light/dark theme toggle using React Context. The selected theme should be stored in localStorage and restored when the page reloads. React focus: use Context to persist theme in localStorage and sync the DOM attribute. Framework focus: React hooks (useState/useEffect), JSX event handling,…

Render a main page layout with a simple content card and a…
Support two themes: "light" and "dark".

What you’ll build / What this tests

This premium react coding focuses on React Theme Toggle with Persisted Light/Dark Mode. You’ll apply react and context thinking with intermediate level constraints. The prompt emphasizes Add a global light/dark theme toggle using React Context. The selected theme should be stored in….

Learning goals

Translate the prompt into a clear react API signature and return shape.
Apply react, context, state techniques to implement react theme toggle with persisted light/dark mode.
Handle intermediate edge cases without sacrificing readability.
Reason about time/space complexity and trade-offs in react.

Key decisions to discuss

Define the exact input/output contract before coding.
Prioritize predictable edge-case handling over micro-optimizations.

Evaluation rubric

Correctness: covers required behaviors and edge cases.
Clarity: readable structure and predictable control flow.
Complexity: avoids unnecessary work for large inputs.
API discipline: no mutation of inputs; returns expected shape.
Testability: solution is easy to unit test.

Constraints / Requirements

Render a main page layout with a simple content card and a theme toggle control in the top-right…
Support two themes: "light" and "dark".
Store the current theme in a React Context provider (global-ish state).
Persist the selected theme in localStorage under a stable key.
On initial load, read the theme from localStorage and apply it before the user interacts.
Apply the theme at the document level by setting data-theme on , so the existing CSS can react…
You only need to edit React/TypeScript files: /src/theme.tsx and /src/App.tsx. The CSS in /src/App.css is already wired to…
By default, the app starts in light mode if there is no previous theme stored.
If the user toggles to dark mode, the page background, text, and card colors change.
Reloading the page keeps the user’s last theme selection (light or dark).

Mini snippet (usage only)

// Example usage
const input = /* react theme toggle with persisted light/dark mode input */;
const result = solve(input);
console.log(result);

// Edge case check
const empty = input ?? null;
const fallback = solve(input);
console.log(fallback);

// Expected: describe output shape, not the implementation
// (no solution code in preview)

Common pitfalls

Mutating inputs instead of returning a new value.
Skipping edge cases like empty input, duplicates, or nulls.
Overlooking time complexity for large inputs.