/* ==========================================
   1. CẤU HÌNH HỆ THỐNG & BIẾN MÀU CẦU VỒNG
   ========================================== */
:root {
    --rainbow: linear-gradient(45deg, #ff007f, #ff7f00, #ffeb3b, #00ff00, #00ffff, #0055ff, #7f00ff);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    color: #ffffff;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background-color: #0f172a; /* Nền tối để làm nổi bật dải màu cầu vồng */
    overflow: hidden;
}

/* ==========================================
   2. KHUNG CHỨA CHÍNH (WELCOME BOX)
   ========================================== */
.welcome-container {
    text-align: center;
    background: #1e293b;
    padding: 45px 35px;
    border-radius: 20px;
    box-shadow: 0 20px 40px rgba(0, 0, 0, 0.5);
    max-width: 400px;
    width: 100%;
    position: relative;
    border: 3px solid transparent;
    background-image: linear-gradient(#1e293b, #1e293b), var(--rainbow);
    background-origin: border-box;
    background-clip: padding-box, border-box;
    background-size: 400% 400%;
    animation: rainbow-animation 8s ease infinite;
}

/* ==========================================
   3. CHỮ CẦU VỒNG & ĐƯỜNG LINK
   ========================================== */
.logo {
    font-size: 3rem;
    font-weight: 900;
    letter-spacing: 2px;
    margin-bottom: 20px;
    background: var(--rainbow);
    background-size: 400% 400%;
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    animation: rainbow-animation 8s ease infinite;
}

.subtitle, p, a {
    color: #94a3b8;
    font-size: 0.95rem;
    line-height: 1.6;
    text-decoration: none;
    transition: color 0.3s;
}

a:hover {
    color: #00ffff; /* Di chuột vào link đổi sang màu xanh neon */
}

/* ==========================================
   4. INPUT NHẬP LIỆU
   ========================================== */
input {
    width: 100%;
    padding: 12px;
    margin-bottom: 15px;
    border-radius: 8px;
    border: 1px solid #334155;
    background: #0f172a;
    color: white;
    outline: none;
    transition: all 0.3s;
}

input:focus {
    border-color: transparent;
    background-image: linear-gradient(#0f172a, #0f172a), var(--rainbow);
    background-origin: border-box;
    background-clip: padding-box, border-box;
}

/* ==========================================
   5. NÚT BẤM CẦU VỒNG
   ========================================== */
.btn {
    display: block;
    width: 100%;
    padding: 14px;
    font-size: 1rem;
    font-weight: 700;
    border-radius: 50px;
    border: none;
    cursor: pointer;
    transition: all 0.3s ease;
    background: var(--rainbow);
    background-size: 300% 100%;
    color: #ffffff;
    box-shadow: 0 4px 15px rgba(255, 0, 127, 0.3);
    text-align: center;
}

.btn:hover {
    background-position: 100% 0;
    transform: translateY(-3px);
    box-shadow: 0 6px 20px rgba(0, 229, 255, 0.5);
}

/* ==========================================
   6. HIỆU ỨNG CHUYỂN ĐỘNG CẦU VỒNG
   ========================================== */
@keyframes rainbow-animation {
    0% { background-position: 0% 50%; }
    50% { background-position: 100% 50%; }
    100% { background-position: 0% 50%; }
}