.preloader {
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    z-index: 9999;
    overflow: hidden;
    display: none;
    width: 50px;
    height: 50px;
    border: 5px solid transparent;
    border-top: 5px solid #fff;
    border-right: 5px solid #fff;
    border-radius: 50%;
    animation: spin 1s linear infinite;
}
@keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(360deg); }
}
.hidden {
    display: none;
}