style.css

*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body{
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: #151320;
}

.caixa{
    position: relative;
    width: 400px;
    height: 400px;
    background-color: #151320; 
    justify-content: center;
    align-items: center;
    border-radius: 3px;
    font: bold 2em 'Century Gothic';
    text-transform: uppercase;
    text-align: center; 
    display: flex;
    align-items: center;
    justify-content: center;
}

p{
    color: antiquewhite;
}

.caixa::before, .caixa::after{
    content: '';
    position: absolute;
    z-index: -1;
    width: calc(100% + 30px);
    height: calc(100% + 30px);
    background: linear-gradient(45deg, #ffff00, #00ff00, #0099ff, #001aff, #a200ff, #ff0055, #ff0000, #ffff00, #00ff00, #0099ff, #001aff, #a200ff);
    background-size: 300%;
    border-radius: 8px;
    animation: Animation 8s linear infinite;
}

.caixa::after{
    filter: blur(50px);
}

@keyframes Animation{
    0%{
        background-position: 0;
    }

    100%{
        background-position: 300%;
    }
}