:root {
    --primary: #2563eb;
    --secondary: #ff4757;
    --dark: #0f172a;
    --light: #f8fafc;
    --transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
}

* { margin: 0; padding: 0; box-sizing: border-box; font-family: 'Inter', sans-serif; }
body { background: var(--light); color: var(--dark); overflow-x: hidden; }

/* HEADER & NAV (Bài 1) */
.header {
    position: sticky; top: 0; z-index: 1000;
    background: rgba(255, 255, 255, 0.95);
    backdrop-filter: blur(10px); box-shadow: 0 2px 15px rgba(0,0,0,0.1);
}
.navbar {
    display: flex; justify-content: space-between; align-items: center;
    padding: 0 8%; height: 80px;
}
.logo { font-size: 1.5rem; font-weight: 800; color: var(--primary); text-decoration: none; }
.nav-links { display: flex; list-style: none; gap: 2rem; }
.nav-links a { 
    text-decoration: none; color: var(--dark); font-weight: 600; 
    transition: var(--transition);
}
.nav-links a:hover, .nav-links a.active { color: var(--primary); }

/* HERO SECTION (Bài 2) */
.hero {
    height: 50vh; display: flex; align-items: center; justify-content: center;
    background: linear-gradient(45deg, #0f172a, #2563eb); color: white; text-align: center;
}

/* CARD HOVER 3D (Bài 2) */
.card-container {
    display: grid; grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 2rem; padding: 5rem 8%;
}
.card {
    background: white; padding: 2.5rem; border-radius: 20px;
    box-shadow: 0 10px 30px rgba(0,0,0,0.05); transition: var(--transition);
    transform-style: preserve-3d; cursor: pointer;
}
.card:hover { transform: rotateY(10deg) rotateX(5deg) translateY(-10px); }

/* MASONRY GRID (Bài 5) */
.masonry { column-count: 3; column-gap: 1.5rem; padding: 2rem 8%; }
.masonry-item { 
    background: #fff; margin-bottom: 1.5rem; border-radius: 12px; 
    break-inside: avoid; overflow: hidden; box-shadow: 0 5px 15px rgba(0,0,0,0.08);
}
.masonry-item img { width: 100%; display: block; transition: 0.4s; }

/* FLOATING LABEL FORM (Bài 4) */
.form-group { position: relative; margin-bottom: 2rem; }
.form-group input, .form-group textarea {
    width: 100%; padding: 12px; border: 2px solid #e2e8f0; border-radius: 10px;
    background: transparent; outline: none; transition: 0.3s;
}
.form-group label {
    position: absolute; left: 15px; top: 12px; color: #94a3b8;
    transition: 0.3s; pointer-events: none;
}
.form-group input:focus ~ label, 
.form-group input:not(:placeholder-shown) ~ label,
.form-group textarea:focus ~ label,
.form-group textarea:not(:placeholder-shown) ~ label {
    top: -25px; left: 0; font-size: 0.8rem; color: var(--primary); font-weight: bold;
}

/* TOAST */
#toast {
    position: fixed; bottom: -100px; right: 20px;
    background: #10b981; color: white; padding: 1rem 2rem;
    border-radius: 10px; transition: 0.5s; z-index: 9999;
}
#toast.show { bottom: 20px; }

/* RESPONSIVE */
@media (max-width: 768px) { .masonry { column-count: 1; } .nav-links { display: none; } }