I wanted to do an actual working plugin this time, but in process of writing I understood that there is a problem.
The current C# SDK isn't really much. There isn't a lot of control. You can't do a plugin that will work with multiple Skins. You will probably end with one static class that will manage most of your skin logic. But the problem with static's is that you can't manage more than one skin.
README from SDK for lazy's
This is my second attempt. Here are Examples and here is SDK - RainManager. See Examples for my shitty explanation. And here is my plugin that I mentioned at the beginning VKPlayer
I'm attaching a picture that will try to explain how the architecture is build.
The architecture is more complex than you expect, be careful and read the source code.
The only reason I'm writing this is that I want to get some feedback. Maybe I have done something wrong, maybe it was possible to do multi skin support with less effort, and this SDK is pointless. I can't really say.
I hope this SDK will be useful for the Rainmeter community.