Based on the info here, you can add the following to your my-theme.css file:
.remark-slide-number {
font-size: 10pt;
margin-bottom: -11.6px;
margin-right: 10px;
color: red; /* white */
opacity: 1; /* default: 0.5 */
}
and play with margin-bottom and margin-right to get it to where you need it.