/* Cookie function */
function getCookie(name) {
    var matches = document.cookie.match(new RegExp(
        "(?:^|; )" + name.replace(/([\.$?*|{}\(\)\[\]\\\/\+^])/g, '\\$1') + "=([^;]*)"
    ));
    return matches ? decodeURIComponent(matches[1]) : undefined;
}

if (getCookie('isVisited')) {
    // меняем CSS
} else {
    // Устанавливаем куки
    document.cookie = 'isVisited=true; path=/';
}

<div class="message-wrapper">
    <div class="message">
        <h2>Test</h2>
    </div>
</div>

.message-wrapper {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: rgba(0, 0, 0, 0.3);
    z-index: 1;
}
.message {
    background-color: #ffffff;
}

$('#mail_user').on('change input', function() {
    // your code
});