body {
    font-family: 'Arial', sans-serif;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    min-height: 100vh;
    margin: 0;
    background-color: #f0f8ff; /* Alice Blue */
    color: #333;
    overflow: hidden; /* To prevent scrollbars if the puzzle is slightly too big */
}

h1 {
    color: #4682b4; /* Steel Blue */
    margin-bottom: 20px;
}

#puzzle-container {
    display: grid;
    grid-template-columns: repeat(4, 100px); /* For a 4x4 puzzle */
    grid-template-rows: repeat(4, 100px);
    width: 400px; /* 4 * 100px */
    height: 400px; /* 4 * 100px */
    border: 5px solid #4682b4; /* Steel Blue */
    border-radius: 10px;
    box-shadow: 0 0 20px rgba(0, 0, 0, 0.2);
    margin-bottom: 30px;
    position: relative; /* For absolute positioning of tiles */
}

.puzzle-tile {
    width: 100px;
    height: 100px;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 2.5em;
    font-weight: bold;
    background-color: #add8e6; /* Light Blue */
    border: 1px solid #4682b4;
    cursor: pointer;
    transition: all 0.2s ease-in-out;
    position: absolute; /* To enable smooth sliding animation */
    box-sizing: border-box; /* Include padding and border in the element's total width and height */
    border-radius: 5px;
    color: #fff; /* White text */
    text-shadow: 2px 2px 4px rgba(0,0,0,0.3);
}

.puzzle-tile:hover {
    background-color: #87ceeb; /* Sky Blue */
    transform: scale(1.05);
}

.empty-tile {
    background-color: transparent; /* Invisible */
    border: none;
    cursor: default;
}

#controls {
    margin-top: 20px;
}

#new-game-btn {
    padding: 12px 25px;
    font-size: 1.2em;
    background-color: #4682b4; /* Steel Blue */
    color: white;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.3s ease;
}

#new-game-btn:hover {
    background-color: #5f9ea0; /* Cadet Blue */
}