Add Accessibility Features & Dark Mode Support

I was exploring **eventstack** and had an idea to make it more user-friendly.

### Suggestions
1. **Accessibility Improvements**
- Add ARIA labels for screen readers
- Ensure all form fields and buttons can be used via keyboard
- Improve color contrast for better readability

2. **Dark Mode Support**
- Add a simple light/dark mode toggle
- Store user preference (e.g., using local storage) so it remembers their choice

### Why this matters
- Accessibility makes the app usable for everyone, including those relying on assistive tools
- Dark mode is now a common expectation and improves comfort, especially for long usage

I’d love to work on this and raise a PR 🚀

Thankyou.