Edit in JSFiddle

.loader {
    border: 6px solid rgba(169,169,169,1); /* Light grey */
    border-radius: 50%;
    width: 280px;
    height: 280px;
}

.loader:hover {
    border-top: 6px solid rgb(33, 129, 198); /* Blue */
    animation: spin 2s normal ease-in 1 forwards;
}

@keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(355deg); }
}