style.css

:root {
    --green: rgba(68,187,6,.5);
    --red: rgba(255,72,64,.5);
    --gray: #ddd;
    --text: #444;
}

* {
    font-family: 'Roboto Mono', monospace;
    font-size: 10px;
    outline: none;
}

.body {
    background-color: var(--gray);
    transition: background-color .4s ease-in;
}

.form {
    margin: 20vh auto 0;
    text-align: center;
}

.title {
    font-size: 4.5rem;
    color: var(--text);
}

.regex {
    border: none;
    border-bottom: 3px solid var(--text);
    background-color: transparent;
    padding: 5px 30px;
    font-size: 3rem;
    color: var(--text);
}

.message {
    font-size: 1.8rem;
}

.submit {
    margin-top: 42px;
    font-size: 2.5rem;
    font-weight: bold;
    padding: 10px 50px;
    border: 3px solid var(--text);
    background-color: transparent;
    color: var(--text);
    transition: transform .3s ease-in;
    cursor: pointer;
}

.submit:hover {
    transform: scale(1.1);
}

.valid .submit,
.invalid .submit {
    margin-top: 0;
}

.valid {
    background-color: var(--green);
}

.invalid {
    background-color: var(--red);
}

.more-info {
    position: fixed;
    bottom: 15px;
    width: 100%;
    text-align: center;
    font-size: 1.3rem;
}

.github-corner {
    background-image: url('ght.png');
    background-size: 110px 110px;
    background-position: center;
    width: 100px;
    height: 100px;
    position: fixed;
    top: 0;
    right: 0;
}