index.css

@import url('https://fonts.googleapis.com/css2?family=DynaPuff&display=swap');
@import url('https://fonts.googleapis.com/css2?family=Nabla&display=swap');

html, body{
    margin: 0;
    padding: 0;
}
body{
    background-color: #1C1C1C;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    font-family: Arial, Helvetica, sans-serif;
}
a{
    color: blueviolet;
}
.conteiner{
    position: relative;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 300px;
    width: 550px;
    border-radius: 15px;
    background-color:#1C1C1C;
    color: white;
    font-size: 25px;
    font-weight: bolder;
    transition: all .5s;
    box-shadow: 1px 3px 3px 2px rgba(1, 1, 1, .7);
    cursor: pointer;
}
.conteiner:hover{
    background-color: #363636;
}
.conteiner::before, .conteiner::after{ 
    content: " ";
    position: absolute;
    z-index: -1;
    width: calc(100% + 30px);
    height: calc(100% + 30px);
    background: linear-gradient(to left, #006400,#7CFC00,#FFFF00,#FF0000,#800000,#8B008B,#4B0082, #00008B, #0000CD,#006400);
    animation: animarborda 10s linear infinite;
    border-radius: 15px;
    background-size: 300%;
    box-shadow: 1px 3px 3px 2px rgba(1, 1, 1, .7);
}
@keyframes animarborda{
    0%{
        background-position: 0;
    }
    100%{
        background-position: 300%;
    }
}
.conteiner::after{
    filter: blur(50px);
}