style.css

@import url('https://fonts.googleapis.com/css2?family=Poppins:wght@300&display=swap');
*{
    margin: 0;
    padding: 0;
    text-decoration: none;
    font-family: 'Poppins', sans-serif;
    box-sizing: border-box;
}
body {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 100%;
    height: 100vh;
}

.container{
    display: flex;
    justify-content: center;
    align-items: center;
    background-color: black;
    flex-direction: column;
    color: #fff;
    padding: 5rem 2rem;
    box-shadow: 0px 0px 20px 0px black;
    border: 1px solid black;
    border-radius: 10px;
    width: 25rem;
}

.container h1{
        font-size: 2rem;
        margin-bottom: 2rem;
}

.userName, .password{
    border: 2px solid cyan;
    padding: 1rem 2rem;
    width: 15rem;
    margin: 1rem;
    background: transparent;
    color: #fff;
    border-radius: 30px;
    font-size: 1.3rem;
    text-align: center;
    outline: none;
    transition: all .2s ease-in-out;
}

.userName:focus,
.password:focus{
    width:20rem;
    border-color: chocolate;
    animation-name: clrChange;
    animation-duration: 10s;
    animation-iteration-count: infinite;
}

@keyframes clrChange {
    0% {
        border-color: green;
        color: green;
    }

    10% {
        border-color: yellow;
        color: yellow;

    }

    20% {
        border-color: hotpink;
        color: hotpink;
    }

    30% {
        border-color: red;
        color: red;
    }

    40% {
        border-color: purple;
        color: purple;
    }

    50% {
        border-color: yellow;
        color: yellow;

    }

    60% {
        border-color: cyan;
        color: cyan;
    }

    70% {
        border-color: skyblue;
        color: skyblue;
    }

    80% {
        border-color: pink;
        color: pink;
    }

    90% {
        border-color: red;
        color: red;
    }

    100% {
        border-color: yellow;
        color: yellow;
    }
}

.loginBtn{
    padding: 1rem 2rem;
    border: 2px solid green;
    background-color: transparent;
    border-radius: 30px;
    margin: 1rem;
    color: #fff;
    font-size: 1rem;
    width: 9rem;
    transition: all .3s ease-in-out;
    cursor: pointer;
}

.loginBtn:hover{
    background-color: green;
}

footer {
    position: fixed;
    left: 0;
    bottom: 0;
    width: 100%;
    color: black;
    text-align: center;
}