<button>Log In</button>

html{
  height:100vh; 
  background: grey; 
  box-sizing: border-box; 
  display: flex;
  align-items: center;
  justify-content: center;
}

button{
  padding: 1rem;
  border: 0.5rem solid rebeccapurple; 
  border-radius: 0.3rem;
  font-size: 1.5rem;
  background: rebeccapurple;
  color: white;
}