Build a notification center that manages read/unread states for multiple notifications. Users can mark individual notifications as read or mark all as read at once.

Requirements:
- Create a container with class `notification-center` that holds all notifications
- Include a header with class `notification-header` containing:
- A title showing "Notifications" text
- An unread count badge with class `unread-count` showing the number of unread notifications
- A "Mark all as read" button with class `mark-all-btn`
- Create exactly 4 notification items, each with class `notification-item`
- Each notification should have class `unread` initially
- Each notification must contain a clickable element with class `mark-read-btn` (can be a button or any element)
- When a notification's `mark-read-btn` is clicked, remove the `unread` class from that notification's `notification-item`
- When the `mark-all-btn` is clicked, remove the `unread` class from ALL notifications
- The `unread-count` badge should dynamically update to show the current number of unread notifications
- Use a monochromatic gray color scheme (different shades of gray, black, and white only)