style.css

body {
    font-family: monospace;
    background-image: linear-gradient(to right, rgb(156, 168, 167), rgb(100, 116, 206));
}

fieldset {
    text-align: center;
    border: 3px solid white;
}

legend {
    border: 1px solid rgb(255, 255, 255);
    padding: 10px;
    text-align: center;
    background-color: rgb(17, 103, 129);
    border-radius: 5px;
    color: rgb(255, 255, 255);
}

.box {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background-color: rgba(0, 0, 0, 0.8);
    padding: 15px;
    border-radius: 15px;
    width: 20%;
    color: white;
}

.inputBox {
    position: relative;
}

.inputUser {
    background: none;
    border: none;
    border-bottom: 1px solid white;
    color: rgb(255, 255, 255);
    font-size: 15px;
    width: 100%;
    letter-spacing: 2px;
}

.inputUser:focus ~ .labelInput, 
.inputUser:valid ~ .labelInput {
    top: -20px;
    font-size: 12px;
    color: rgb(83, 190, 149);
}

#submit {
    background-image: linear-gradient(to right, rgb(8, 253, 221), rgb(100, 116, 206));
    width: 100%;
    border: none;
    padding: 15px;
    color:rgb(255, 255, 255);
    font-size: 15px;
    cursor: pointer;
    border-radius: 10px;
    font-family: monospace;
}

#submit:hover {
    background-image: linear-gradient(to right, rgb(177, 75, 168), rgb(185, 172, 54));
}