1. Go to the active CSS file and add there the new styles you would like to use, for example:
.offerBlue .MiddleCenter { background-color: blue; }
.offerBlue .offerText { font-size: 20px; }
.offerRed .MiddleCenter { background-color: red; }
.offerRed .offerText { font-size: 20px; }
2. Go to CMS settings of Special Offer block and specify one of the CSS classes you've created 'offerBlue' or 'offerRed'.