Indeed, as RadChart is an image rendered on the server, CSS is not applicable. Still, you can create your own chart appearance set (skin) and save it in an .xml file for later reuse. This is described in details in this help topic
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items