Improve the ThemeManager to allow switching styles

Currently the `ThemeManager` allows to change the Theme and the Colors but as we have more and more styles for non-Metro I think it would be nice to have a new switch/selector that would allow to change more easily between `Metro`, `Win10` and `MUI`. I don't think this change would happen dynamically inside the app on the contrary to the theme and colors but it would be convenient to just call one method to have all default styles set to the right "theme".

WDYT?

Read the ThemeManager implementation and its existing theme and color switching APIs first; compare how Metro, Win10, and MUI styles are represented. Clarify whether the proposed selector is initialization-only, then define completion as one public call that selects each style set and verifies the corresponding defaults are applied.

Written by the indexing model from the issue text.