style.css

/* Center the div .container */
/* Center all the elements in it */
.container {
  position: absolute;
  top: 55%;
  left: 50%;
  transform: translate(-50%, -50%);
  display: flex;
  flex-direction: column;
  align-items: center;
}

#control {
    margin: 2em;
    padding: .5em;
    width: 80%;
}

#submit {
  margin: 1em;
  display: flex;
  justify-content: center;
  align-items: center;
}

.btn, .btn2{
     width: 250px;
     height: 50px;
     font-size: 30px;
     text-align: center;
     line-height: 100px;
     color: rgba(255,255,255,0.9);
     border-radius: 50px;
     background: linear-gradient(-45deg, #FFA63D, #FF3D77, #338AFF, #3CF0C5);
     background-size: 600%;
     animation: anime 16s linear infinite;
  }

.btn2{
     position: absolute;
     margin-top: -70px;
     z-index: -1;
     filter: blur(30px);
     opacity: 0.8;
  }
     
 @keyframes anime {
   0%{
     background-position: 0% 50%;
   }
   50%{
     background-position: 100% 50%;
   }
   100%{
     background-position: 0% 50%;
   }
 }