.center {
    position: absolute;
    top: 50%;
    left: 50%;
    margin: 0 -50% 0 0;
    transform: translate(-50%, -50%)
}

.loader {
    width: 25px;
    height: 25px;
    border: 5px solid #3b3b3b;
    border-bottom-color: transparent;
    border-radius: 50%;
    display: inline-block;
    box-sizing: border-box;
    animation: rotation 1s linear infinite;
}

@keyframes rotation {
    0% {
        transform: rotate(0deg);
    }
    100% {
        transform: rotate(360deg);
    }
}