style.css

.grid {
  width: 200px;
  height: 200px;
  display: flex;
  flex-wrap: wrap;
  border-style: solid;
  background-color: #0c002b;
  position: absolute;
  top: 50%;
  left: 50%;
  visibility: hidden;
  transform: translate(-50%, -50%);
}

.grid div {
  width: 20px;
  height: 20px;
}

.score{
  text-align: center;
}

.snake {
  background-color: green;
}

.apple {
  background-color: purple;
}

#btn {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  color: #f3f3f3;
  text-decoration: none;
  font-size: 5vw;
  padding: 2rem 4rem;
  box-shadow: 0 30px 50px #1a0749;
  overflow: hidden;
  background-color: #0c002b;
}

#btn span:nth-child(1) {
  position: absolute;
  top: 0;
  left: 0;
  height: 3px;
  width: 100%;
  background: linear-gradient(to right, #0c002b, #1779ff);
  animation: animate1 2s linear infinite;
}

@keyframes animate1 {
  from {
    transform: translateX(-100%);
  }
  to {
    transform: translateX(100%);
  }
}

#btn span:nth-child(2) {
  position: absolute;
  top: 0;
  right: 0;
  width: 2px;
  height: 100%;
  background: linear-gradient(to bottom, #0c002b, #1779ff);
  animation: animate2 2s linear infinite;
  animation-delay: 1s;
}

@keyframes animate2 {
  from {
    transform: translateY(-100%);
  }
  to {
    transform: translateY(100%);
  }
}

#btn span:nth-child(3) {
  position: absolute;
  bottom: 0;
  right: 0;
  height: 2px;
  width: 100%;
  background: linear-gradient(to left, #0c002b, #1779ff);
  animation: animate3 2s linear infinite;
}

@keyframes animate3 {
  from {
    transform: translateX(100%);
  }
  to {
    transform: translateX(-100%);
  }
}

#btn span:nth-child(4) {
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 2px;
  background: linear-gradient(to top, #0c002b, #1779ff);
  animation: animate4 2s linear infinite;
  animation-delay: 1s;
}
@keyframes animate4 {
  from {
    transform: translateY(100%);
  }
  to {
    transform: translateY(-100%);
  }
}