Add image to ribbon Change ribbon color

I would like to add a picture to my ribbon – how can I do this? I would also like to know how I change the colors on the ribbons.

I put this in the CSS but nothing happened:

.separator-one {
background-color: rgba( 234, 35, 38, 0.5 );
}
purchase-now {
background-color: rgba( 234, 35, 38, 0.5 );
}