Fix theme toggle button not working due to SSR hydration mismatch

[Copilot]

The theme mode toggle button in the header dropdown was not working due to a Next.js server-side rendering (SSR) hydration mismatch in the ThemeContext component.

Problem

The ThemeContext was accessing localStorage during the initial render without checking if it was running on the client side. This caused hydration mismatches between what the server rendered and what the client expected, preventing the theme toggle functionality from working properly.

Solution

Updated the ThemeContext to properly handle SSR by:

1. Added client-side safety checks: Wrapped all localStorage access with typeof window !== 'undefined' checks
2. Implemented mounted state: Added a mounted state to prevent theme application until the component has hydrated on the client
3. Delayed theme initialization: Theme detection and application now only occurs after the component has mounted

Changes

// Before: Direct localStorage access during SSR
const savedTheme = localStorage.getItem('theme') as Theme;

// After: SSR-safe localStorage access
if (typeof window !== 'undefined') {
  const savedTheme = localStorage.getItem('theme') as Theme;
  // ...
}

Testing

The fix has been verified to work correctly:

• ✅ Theme toggle switches between light and dark modes
• ✅ Visual changes apply properly (backgrounds, text colors, UI elements)
• ✅ Theme preference persists across page refreshes
• ✅ No hydration warnings or console errors
• ✅ SSR-compatible implementation

Screenshots

Light Mode:

Dark Mode:

The theme mode button in the header dropdown now works as expected for all authenticated users, allowing seamless switching between light and dark themes.

Fixes #207.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.