Make IModelDecorationOptions/CssStyles XAML Friendly

[hawkerm]

I'd be cool if we could make these styles more XAML resource dictionary friendly (e.g. convert to DependencyObjects?). This would make defining, copying, and re-using them a bit easier.

Though since the API would always be called from the code-behind, it'd be a bit awkward to use still?

Curious if this makes sense in the end...? Thoughts?