Blame for static/assets/css/loader.css

.loader {
    border: 8px solid #f3f3f3; /* Light gray */
    border-top: 8px solid #3498db; /* Blue */
    border-radius: 50%;
    width: 50px;
    height: 50px;
    animation: spin 1s linear infinite;
    position: fixed;
    top: 50%;
    left: 50%;
    margin-top: -25px;
    margin-left: -25px;
    display: none;
}

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