I've reviewed the help topics on Creating a Custom Theme, but I'm not sure how to solve the following two problems:

1)  I allow for the use of different Telerik themes within my application.  However, I've also created some custom controls which I would like to participate in your theme system so my application maintains a consistent look and feel.  (Most of these are very simple controls such as buttons that draw icons in XAML -- I'd like the foreground colors to match the current theme.)  How can I make my custom controls participate in the different Telerik themes?

2)  I'd like to modify certain very specific parts of your existing themes.  For example, I want to set the cursor color of the TextBox to have a TemplateBinding to the Foreground property.  What's the best way to do this?  Can I derive a custom theme off of one of your existing themes and make this targeted change or should I extract the existing XAML for each theme/control combination and rebuild the existing theme?

Oh and it would be nice if the solution was reusable across Silverlight and WPF -- I have a composite SL/WPF app.

[edit] Some clarifications, based on reading other forum posts about themes.  Theme selection is done via configuration files, but not at runtime.  This is not a dynamic resource issue.  For color selection of my custom controls, it would be nice if I could use a predefined Telerik resource, however, I would be satisfied if I could at least define my own color or brush resources for a given theme and then use them in my controls.