Presently, the color blending feature is not available and the easiest way to modify your theme is by using the Visual Style Builder. We plan to add the color blending feature in future, however it is not yet scheduled and I cannot give you a time frame when it will be implemented.
If you still want to do this with code, you can extract images from your theme by using the following code:
Theme theme = ThemeRepository.FindTheme(
Bitmap bmp = setting.Value
// do something with the bitmap
I hope this helps.
the Telerik team