Hi Thomas Luck,
There are some differences between RadChart and most of the other controls when it comes to skinning. Since RadChart is just an image which is generated on the server, there is no CSS, but every skin is a set of predefined appearance properties, which are applied to the chart.
However I suggest you to use the following approach
1. modify some of the existing skin's properties and
2. apply the generated .xml file using the ApplyTo method as shown here.
I hope this helps.
Best wishes,
Sia
the Telerik team