html,
body {
    height: 100%;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100%;
    background: #1D1F20;
}

#box {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 400px;
    height: 200px;
    color: white;
    font-family: 'Times New Roman';
    font-size: 2.5rem;
}

.gradient-border {
    --borderWidth: 3px;
    background: #1D1F20;
    position: relative;
    border-radius: var(--borderWidth);
}

.gradient-border:after {
    content: '';
    position: absolute;
    top: calc(-1 * var(--borderWidth));
    left: calc(-1 * var(--borderWidth));
    height: calc(100% + var(--borderWidth) * 2);
    width: calc(100% + var(--borderWidth) * 2);
    background: linear-gradient(60deg, #f79533, #f37055, #ef4e7b, #a166ab, #5073b8, #1098ad, #07b39b, #6fba82);
    border-radius: calc(2 * var(--borderWidth));
    z-index: -1;
    animation: animatedgradient 3s ease alternate infinite;
    background-size: 300% 300%;
}


@keyframes animatedgradient {
    0% {
        background-position: 0% 50%;
    }

    50% {
        background-position: 100% 50%;
    }

    100% {
        background-position: 0% 50%;
    }
}