Enhance styling of components

Declaring styles separately from component hierarchy can help make the code more comprehensible.

Some challenges when applying styles to components include:
- Widget.apply() only applies to children of the widget. When creating a component, one would like to style the component itself as well.
- Styles sometimes differ among platforms. What would be a lean way to declare platform-specific styles?
- How should styling of dynamically created items be handled? (e.g. CollectionView cells)