static/index.css

.loader {
    border: 16px solid #f3f3f3;
    /* Light grey */
    border-top: 16px solid #3498db;
    /* Blue */
    border-radius: 50%;
    width: 120px;
    height: 120px;
    animation: spin 2s linear infinite;
    display: none;
    /* Initially hidden */
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

@keyframes spin {
    0% {
        transform: rotate(0deg);
    }

    100% {
        transform: rotate(360deg);
    }
}