style/style.css

:root {
    --color-env: #c41e3a;
    --color-env2: #a01729;
    --color-flap: #2e7d32;
    --color-bg: #165016;
    --color-heart: #ff0000;
    --color-sparkle: #fff;
    --wax-red: #c41e3a;
}

body {
    background: radial-gradient(circle at top, #1f1f41, #18101b 80%);
    margin: 0;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    align-items: center;
    user-select: none;
    cursor: auto;
    position: relative;
    overflow: hidden; 
}

.envlope-wrapper {
    height: 380px;
    margin-top: 50px;
    animation: float 3s ease-in-out infinite;
}

#envelope {
    position: relative;
    width: 380px;
    height: 240px;
    border-bottom-left-radius: 6px;
    border-bottom-right-radius: 6px;
    margin: 0 auto;
    top: 150px;
    background-color: var(--color-flap);
    box-shadow: 0 4px 20px rgba(0,0,0,.1);
    cursor: pointer;
}

.front {
    position: absolute;
    width: 0;
    height: 0;
    z-index: 3;
}

.flap {
    border-left: 190px solid transparent;
    border-right: 190px solid transparent;
    border-bottom: 92px solid transparent;
    border-top: 150px solid var(--color-flap);
    transform-origin: top;
    pointer-events: none;
}

.pocket {
    border-left: 190px solid var(--color-env);
    border-right: 190px solid var(--color-env);
    border-bottom: 90px solid var(--color-env2);
    border-top: 150px solid transparent;
    border-bottom-left-radius: 6px;
    border-bottom-right-radius: 6px;
}

.letter {
    position: relative;
    background-color: #e0c784;
    width: 95%;
    margin: 0 auto;
    height: 95%;
    top: 0%;
    border-radius: 15px;
    box-shadow: 0 2px 26px rgba(0,0,0,.08);
    padding: 15px;
    box-sizing: border-box;
    overflow: hidden;
    border: 6px solid;
    border-image: repeating-linear-gradient(
        45deg,
        #ff0000 0 10px,
        #ffffff 10px 20px
    ) 1;
}

.letter:after {
    content: '';
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    background-image: linear-gradient(180deg, 
        rgba(255,255,255,0.00) 25%, 
        rgba(255,231,236,0.70) 55%, 
        rgba(255,231,236,1.00) 100%);
    pointer-events: none;
}

.message {
    position: relative;
    z-index: 2;
    font-family: 'Pacifico', cursive;
    color: #054508;
    text-align: center;
    line-height: 1.4;
    padding: 5px;
    height: 100%;
    overflow-y: auto;
}

.message p {
    margin: 10px 0;
    font-size: 1.4em;
    text-shadow: 0 2px 3px rgba(0,0,0,0.1);
}

.lyrics {
    display: none;
    font-size: 15px;
    line-height: 1.5;
    text-align: center;
    opacity: 0;
    transition: opacity 1s ease-in;
    max-height: 130px;
    overflow-y: auto;
}

.lyric-page {
    display: none;
    opacity: 0;
    transition: opacity 0.5s ease;
}

.lyric-page.active {
    display: block;
    opacity: 1;
}

.open .flap {
    transform: rotateX(180deg);
    transition: transform 0.4s ease, z-index 0.6s;
    z-index: 1;
}

.close .flap {
    transform: rotateX(0deg);
    transition: transform 0.4s 0.6s ease, z-index 1s;
    z-index: 5;
}

.close .letter {
    transform: translateY(0px); 
    transition: transform 0.4s ease, z-index 1s;
    z-index: 1;
}

.open .letter {
    transform: translateY(-80px) rotate(-3deg);
    transition: transform 0.4s 0.6s ease, z-index 0.6s;
    z-index: 2;
}

.letter-corner {
    position: absolute;
    width: 20px;
    height: 20px;
    border: 2px solid #2a470c;
    border-radius: 5px;
    z-index: 3;
}

.corner-tl { top: 10px; left: 10px; border-right: none; border-bottom: none; }
.corner-br { bottom: 10px; right: 10px; border-left: none; border-top: none; }

.hearts, .sparkles {
    position: absolute;
    top: 90px;
    left: 0;
    right: 0;
    z-index: 2;
}

.heart, .sparkle {
    position: absolute;
    bottom: 0;
    pointer-events: none;
}

.heart:before,
.heart:after {
    content: "";
    position: absolute;
    left: 25px;
    top: 0;
    width: 25px;
    height: 40px;
    background: var(--color-heart);
    border-radius: 25px 25px 0 0;
    transform: rotate(-45deg);
    transform-origin: 0 100%;
}

.heart:after {
    left: 0;
    transform: rotate(45deg);
    transform-origin: 100% 100%;
}

.sparkle {
    width: 8px;
    height: 8px;
    background: var(--color-sparkle);
    border-radius: 50%;
    animation: sparkleTwinkle 1s infinite;
}

.close .heart,
.close .sparkle {
    opacity: 0;
    animation: none;
}

.a1 { left: 20%; transform: scale(0.6); animation: slideUp 4s linear infinite, sideSway 2s ease-in-out infinite alternate; }
.a2 { left: 55%; animation: slideUp 5s linear infinite, sideSway 4s ease-in-out infinite alternate; }
.a3 { left: 10%; transform: scale(0.8); animation: slideUp 7s linear infinite, sideSway 2s ease-in-out infinite alternate; }

.s1 { left: 30%; animation: sparkleUp 3s linear infinite; }
.s2 { left: 60%; animation: sparkleUp 4s linear infinite; }
.s3 { left: 45%; animation: sparkleUp 5s linear infinite; }

@keyframes slideUp {
    0% { top: 0; }
    100% { top: -600px; }
}

@keyframes sideSway {
    0% { margin-left: 0; }
    50% { margin-left: 50px; }
    100% { margin-left: 0; }
}

@keyframes sparkleUp {
    0% { transform: translateY(0); opacity: 1; }
    100% { transform: translateY(-500px) rotate(360deg); opacity: 0; }
}

@keyframes sparkleTwinkle {
    0%, 100% { transform: scale(1); }
    50% { transform: scale(1.5); }
}

.controls {
    text-align: center;
    margin-top: 50px;
}

.controls button {
    font-weight: 600;
    background: linear-gradient(135deg, #c41e3a, #a01729, #2e7d32);
    border: 2px solid #ffd700;
    border-radius: 25px;
    color: rgb(255, 255, 255);
    padding: 12px 28px;
    margin: 10px;
    font-size: 17px;
    cursor: pointer;
    font-family: 'Poppins', sans-serif;
    
    /* Hiệu ứng Noel */
    box-shadow: 0 4px 15px rgba(0,0,0,0.2),
                0 0 10px rgba(255, 215, 0, 0.4);
    transition: all 0.35s ease;
    letter-spacing: 0.5px;
}
.controls button:hover {
    /* Tăng độ sáng và glow */
    transform: translateY(-4px) scale(1.07);
    background: linear-gradient(135deg, #ff003c, #c41e3a, #3fa34d);
    box-shadow: 0 6px 25px rgba(0,0,0,0.25),
                0 0 14px rgba(255, 215, 0, 0.6),
                0 0 25px rgba(255, 0, 0, 0.4);
}

/* Nhấn xuống */
.controls button:active {
    transform: scale(0.96);
    box-shadow: 0 3px 12px rgba(0,0,0,0.2);
}

.open .lyrics {
    display: block;
    opacity: 1;
}

.page-indicator {
    position: absolute;
    bottom: 5px;
    left: 0;
    right: 0;
    text-align: center;
    font-size: 0.8em;
    color: #274715;
    opacity: 0.7;
}

/* ====== HIỆU ỨNG NGÔI SAO LẤP LÁNH ====== */
.star-field {
    position: fixed;
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    overflow: hidden;
    pointer-events: none; /* Không cản click */
}

.star {
    position: absolute;
    width: 3px;
    height: 3px;
    background: white;
    border-radius: 50%;
    opacity: 0;
    animation: twinkle 2s infinite ease-in-out;
}

@keyframes twinkle {
    0% { opacity: 0; transform: scale(0.5); }
    50% { opacity: 1; transform: scale(1.5); }
    100% { opacity: 0; transform: scale(0.5); }
}
/* ====== HIỆU ỨNG BÔNG TUYẾT RƠI ====== */
.snowflake {
    position: fixed;
    top: -10px;
    color: white;
    font-size: 12px;
    opacity: 0.8;
    pointer-events: none;
    animation: fall linear infinite;
}

@keyframes fall {
    0% { transform: translateY(0) rotate(0deg); opacity: 0.9; }
    100% { transform: translateY(100vh) rotate(360deg); opacity: 0.2; }
}
.santa {
  position: fixed;
  bottom: 5%;       
  right: -400px;     
  width: 400px;       
  animation: flySanta 12s linear infinite;
  z-index: 9999;
}

@media (max-width: 768px) {
  .santa {
    top: 50%;   /* thay đổi theo ý bạn */
    width: 250px;  /* optional: cho vừa màn điện thoại */
    right: -250px; /* optional: chỉnh theo width */
  }
}

@keyframes flySanta {
  0% {
    transform: translateX(0);
  }
  100% {
    transform: translateX(-150vw); /* Bay ra khỏi mép trái */
  }
}