* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
#header {
  text-align: center;
}
.btn {
  font-weight: bold;
  margin: 1rem;
  padding: 7px 10px;
  border-radius: 5px;
  outline: none;
  transition: all 0.5s;
}
.change-color {
  background: black;
  color: white;
}
.container {
  margin: 1rem auto;
  width: 600px;
  height: 600px;
}