/* Modern Dark Theme */
body { 
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif; 
    display: flex; 
    flex-direction: column; 
    align-items: center; 
    justify-content: center;
    min-height: 100vh;
    margin: 0;
    padding: 20px; 
    background: radial-gradient(circle at center, #1a1a2e 0%, #0f0f1a 100%);
    color: white;
}

/* Glassmorphism Container */
.container { 
    background: rgba(255, 255, 255, 0.05); 
    padding: 40px; 
    border-radius: 20px; 
    box-shadow: 0 15px 35px rgba(0,0,0,0.5); 
    text-align: center; 
    border: 1px solid rgba(255, 255, 255, 0.1);
    backdrop-filter: blur(10px);
    max-width: 90%;
}

h1 {
    margin-top: 0;
    background: linear-gradient(to right, #00c9ff, #92fe9d);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    font-size: 1.8rem;
}

/* Hidden File Input with Custom Label */
#upload { display: none; }
.upload-label {
    display: inline-block;
    padding: 12px 25px;
    background: rgba(255, 255, 255, 0.1);
    border: 1px dashed rgba(255, 255, 255, 0.3);
    border-radius: 10px;
    cursor: pointer;
    transition: all 0.3s ease;
    margin-bottom: 20px;
}
.upload-label:hover { background: rgba(255, 255, 255, 0.2); }

/* Checkerboard Canvas */
canvas { 
    border-radius: 10px;
    margin-top: 20px; 
    max-width: 100%; 
    height: auto;
    box-shadow: 0 0 20px rgba(0,0,0,0.3);
    /* Traditional transparency checkerboard pattern */
    background-image: 
        linear-gradient(45deg, #222 25%, transparent 25%), 
        linear-gradient(-45deg, #222 25%, transparent 25%),
        linear-gradient(45deg, transparent 75%, #222 75%), 
        linear-gradient(-45deg, transparent 75%, #222 75%);
    background-size: 20px 20px; 
    background-position: 0 0, 0 10px, 10px -10px, -10px 0px;
    background-color: #333; 
}

.controls { 
    margin: 25px 0;
    background: rgba(0,0,0,0.2);
    padding: 15px;
    border-radius: 12px;
}

/* Modern Range Input */
input[type=range] {
    cursor: pointer;
    width: 200px;
    accent-color: #00c9ff;
}

#opacityValue {
    font-weight: bold;
    color: #00c9ff;
    margin-left: 10px;
}

/* Action Button */
button { 
    padding: 15px 30px; 
    cursor: pointer; 
    background: linear-gradient(135deg, #00c9ff 0%, #007bff 100%);
    color: white; 
    border: none; 
    border-radius: 50px; 
    font-weight: bold;
    font-size: 1rem;
    transition: transform 0.2s, box-shadow 0.2s;
    box-shadow: 0 4px 15px rgba(0, 201, 255, 0.3);
}

button:hover { 
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(0, 201, 255, 0.5);
}

button:active { transform: translateY(0); }