style.css

html {
    background-color: #181818;
    font-family: Arial, Helvetica, sans-serif;
    color: white;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    margin: 0;
}

form {
    width: 90%;
    min-width: 28.125em;
    max-width: 35em;
    background-color: #0e1010;
    padding: 1em;
}

fieldset {
    border: none;
    padding: 0;
    margin: 0;
}

label,
input,
textarea {
    display: block;
    width: 100%;
    box-sizing: border-box;
}

input,
textarea {
    background-color: rgba(0, 0, 0, 0.3);
    border: 2px solid transparent;
    border-bottom-color: #bbb;
    padding: 0.75em;
    margin: 0.5em 0 2.5em;
    color: white;
    outline: 0;
}

/* input:invalid,
textarea:invalid {
    background-color: red;
} */

/* input:valid,
textarea:valid {
    background-color: green;
} */

input:invalid:focus,
textarea:invalid:focus {
    border-bottom: 2px solid #ff000d;
}

input:valid:focus,
textarea:valid:focus {
    border-bottom: 2px solid #0ec72a;
}

input:focus,
textarea:focus {
    border-bottom: 2px solid #8e999a;
    background-color: #2a2727;
}

legend {
    padding: 0.5em;
    text-align: center;
    font-weight: bold;
    color: #8e999a;
    font-size: 1.3em;
}

.btn {
    background-color: #2a2727;
    color: white;
    cursor: pointer;
    padding: 0.7em;
    font-size: 1.2em;
    border: none;
}