Frankly, we have not tried this approach and therefore we have no proper articles or guidelines on how to do accomplish this goal. I checked with our devs and they said that this would not be so easy to achieve.
There is no dedicated ITheme interface that could help you in this specific case. Additionally, our Theme assemblies are just containers for XAML files, there are no classes defined inside them, hence I suppose MEF will not be able to load them automatically. You could give this approach a go and let us know if you face certain difficulties.
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