Update Themes article for Fusion

[platosha]

The Design System / Customisation / Themes article in some parts does not apply to Fusion views:

• Changing themeClass in @Theme does not apply to Fusion views, it is required to change paths to @vaadin/vaadin-button/theme/material/vaadin-button to apply Material theme to Vaadin components.
  • For @NoTheme, this limitation is mentioned already.
• applyTheme is needed in custom web components, and Fusion views that use Shadow DOM, to apply Lumo styles or descendant theme declared by @Theme
• Theme resolution notes also only apply to Flow views

[jouni]

We’re updating the content of those articles this week. Here’s the draft: https://docs.google.com/document/d/1h8hpkCrBJ3asrET-Cj4d8fOGomKR9EfHIMWR9RDjvzI/edit#