body {
display: flex;
justify-content: center;
align-items: center;
height: 100vh;
margin: 0;
}
button {
padding: 3rem;
background-color: yellow;
border: none;
transition-duration: 600ms;
transition-timing-function: ease;
transition-properties: border-radius;
}
button:hover {
border-radius: 2rem;
}
div {
background-color: blue;
position: fixed;
top: 0;
left: 0;
width: 100%;
height: 100%;
z-index: -1;
transition: background-color 600ms ease;
}
button:hover + div {
background-color: springgreen;
}
index.css