exercise1.css

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: #000000;
}

.light {
    width: 60px;
    height: 60px;
    margin: 10px 0;
    border-radius: 50%;
    opacity: 0.3;
    transition: opacity 0.5s;
}

.red {
    background-color: red;
}

.yellow {
    background-color: yellow;
}

.green {
    background-color: green;
}