Vibe Code a dark mode toggle Feature in Swift

You are an expert software engineer with deep expertise in Swift development.

Let's vibe code a dark mode toggle feature together using Swift. I want you to act as my AI coding partner -- think out loud, suggest approaches, and write the code with me.

**Feature:** dark mode toggle
**Language:** Swift
**Approach:** outside-in development
**Tone:** Collaborative, move fast, explain as we go

## Context
We are adding a dark mode toggle feature to an existing application. The codebase uses Swift and follows standard conventions for that ecosystem.

## Step 1: Design the Interface
- Define the public API or interface for this feature
- List the inputs, outputs, and side effects
- Identify edge cases we need to handle from the start
- Sketch out the type signatures or function prototypes

## Step 2: Implement the Core Logic
- Write the main function or class that powers this feature
- Use idiomatic Swift patterns and best practices
- Add inline comments explaining non-obvious decisions
- Handle the happy path first, then layer in error handling

## Step 3: Add Error Handling and Validation
- Input validation with descriptive error messages
- Graceful degradation for network or I/O failures
- Logging at appropriate levels (debug, info, warn, error)
- Custom error types or error codes for different failure modes

## Step 4: Write Tests
- Unit tests for the core logic (at least 5 test cases)
- Edge case tests (empty input, null values, boundary conditions)
- Integration test for the full feature workflow
- Describe what a performance test would look like

## Step 5: Refactor and Document
- Review the code for duplication and extract shared utilities
- Add docstrings or JSDoc comments to all public functions
- Write a brief usage guide showing how to call this feature
- List any known limitations or future improvement ideas

## Vibe Coding Rules
- Favor working code over perfect code
- If stuck for more than 2 minutes on an approach, try a different one
- Explain trade-offs as you make decisions
- Keep functions under 30 lines where possible

Present as numbered steps. Each step should have: a clear action title, detailed instructions, expected outcome, and common pitfalls to avoid.