*::-webkit-input-placeholder {
    color: #333;
    opacity: 1;
}

*:-moz-placeholder {
    color: #333;
    opacity: 1;
}

*::-moz-placeholder {
    color: #333;
    opacity: 1;
}

*:-ms-input-placeholder {
    color: #333;
    opacity: 1;
}

:root {
    --white: #fff;
    --gray: #666;
    --black: #000;
    --tr: transparent;
    --fs-10: 1.0rem;
    --fs-11: 1.1rem;
    --fs-12: 1.2rem;
    --fs-14: 1.4rem;
    --fs-16: 1.6rem;
    --fs-18: 1.8rem;
    --fs-20: 2rem;
}

body {
    background-image: url(../img/background.jpg);
    background-color: var(--black);
    background-repeat: no-repeat;
    background-position: center center;
    background-attachment: fixed;
    background-size: cover;
}

.center {
     position: absolute;
     display: block;
     width: 200px;
     bottom: 80px;
     left: 50%;
     margin-left: -100px;
     text-align: center;
}
.center a {
     color: var(--white);
     text-decoration: none;
     border: 1px solid var(--white);
     padding: 10px 20px;
     border-radius: 15px;
}
.center a:hover {
     color: var(--black);
     background-color: var(--white);
     border: 1px solid var(--tr);
}