style.css

:root{
  --bg-color:green;
}

body{
  height: 100vh;
  width: 100vw;
  border: none;
  padding: 0;
  margin: 0;  
  background-color: var(--bg-color);
  font-size: 16px;
  box-sizing:border-box;
}
.navbar{
  height: 2rem;
  padding:0;
  margin:0;
  display: flex;
  flex-direction: row;
  justify-content: space-around;
  align-items: center;
  background-color: whitesmoke;
  font-size: 1.5rem;
}

li{
  border-right: 2px black solid;
}
li:last-of-type{
  border: none;
}
#simple{
  text-decoration: underline;
  cursor: pointer;
}
#rgb{
  text-decoration: none;
  cursor: pointer;
}
#hex{
  text-decoration: none;
  cursor: pointer;
}

ul, li{
  display:inline;
  padding:0 1rem;
  margin: 0;
}

section{
  padding: 0 15rem;
  height: 90vh;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items:center;
}

h1{
  font-size: 3rem;
  background-color: black;
  color:whitesmoke;
  border-radius: 15px;
  padding:10px;
  margin-bottom:25px;
}
h1>span{
  color: var(--bg-color);
}

button#change-color{
  background-color: black;
  outline: none;
  border: black 2px solid;
  border-radius: 15px;
  color: white;
  padding: 15px;
  margin-top: 25px;
  cursor: pointer;
  font-size: 18px;
}

button#change-color:hover{
  border: #aaa 2px solid;
}

button#change-color:active{
  background-color: #555;
}