The default OfficeBlack theme, included in the Charting dll is applied by the default Silverlight engine (using styles and templates). Changing the theme in Silverlight 2.0 at runtime is not a simple process, it requires additional theming engine, which we implemented. That engine, unfortunately, sets additional restrictions on the XAML used and currently requires that all resources it will manage, to be in a single assembly.
The Silverlight 3.0 styling will render this engine obsolete as it allows simple theme changing the way WPF allows (e.g. in Silverlight 2.0, change of control style can be done once only,while 3.0 allows multiple changes in style).
Each theme created in Telerik is complete and goes through a process of testing to confirm coherency.
On the problem with the Summer theme, I assure you, the labels are not left with default values. There is an issue with applying the theme, which we are working to resolve.
the Telerik team