html, body {
    background-color:black;
    display: flex; 
    justify-content: center;
    align-items: center;
    /* overflow: hidden;
    transform: scale(1.5); */
}

/* button */
.btn {

    padding: 15px 40px;
    border: none;
    outline: none;
    color: #fff;
    cursor: pointer;
    position: relative;
    z-index: 1;
    border-radius: 12px;
}

.btn::after {
    content: "";
    z-index: -1;
    position: absolute;
    width: 100%;
    height: 100%;
    background-color: #333;
    left: 0;
    top: 0;
    border-radius: 10px;
}

/* Glow */
.btn::before {
    content: "";
    background: linear-Gradient(45deg, #FF0000, #FF7300, #FFFB00, #48FF00, #00FFD5, #002BFF, #FF00C8, #FF0000);

position: absolute;
top: -2px;
left: -2px;
background-size: 600%;
z-index: -1;
width: calc(100% + 4px);
height: calc(100% + 4px);
filter: blur(8px);
animation: glowing 20s linear infinite;
transition: opacity .3s ease-in-out;
border-radius: 10px;
opacity: 0;
}

@keyframes glowing {
    0% {background-position: 0 0;}
    50% {background-position: 400% 0;}
    100% {background-position: 0 0;}
}
/* hoover */
.btn:hover::before {
    opacity:1;
}

.btn:active::after {
    background: transparent;
}

.btn:active {
    color:#000;
    font-weight: bold;
}