body {
    margin: 0;
    background: #000;
    font-family: Arial, sans-serif;
}

.container {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    height: 100vh;
}

h1 {
    font-size: 4rem;
    margin-bottom: 1rem;
    color: #fff;
}

.envelope {
    width: 60%;
    height: 80%;
    perspective: 1000px;
}

.envelope-inner {
    width: 100%;
    height: 100%;
    position: relative;
    transform-style: preserve-3d;
    transition: transform 1s;
}

.front, .back {
    width: 100%;
    height: 100%;
    position: absolute;
    backface-visibility: hidden;
}

.front {
    background: #FFF;
    z-index: 2;
}

.back {
    background: #348feb;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    text-align: center;
    color: #fff;
    transform: rotateY(180deg);
}

h1 {
    font-size: 4rem;
    margin-bottom: 1rem;
}

.flipped .envelope-inner {
    transform: rotateY(180deg);
}

#fireworks {
    position: absolute;
    top: 0;
    left: 0;
    z-index: -1;
}