Some skins do not support declarative customization of the cell's background color, because the skin uses background images and they take precedence over background colors.
I will have to ask you to send your custom skin, but an easier solution for you will be to use the following CSS rules:
.RadCalendar_SkinName .rcMainTable .rcToday
.RadCalendar_SkinName .rcMainTable .rcToday a
If you still have problems, then please send your custom skin.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.