[tutorial] Shadow DOM styling

Nobody has claimed this yet.

- If you're playing along at home, you've already met scoped styles in our intro tutorial, right?
- Styling the host with `:host`
- Conditionally styling the host with `:host()`
- And a digression about properties with `reflect: true`.
- Or allowing users to set classes/attributes from the outside.
- Maybe some discussion of whether you should set classes on the host, or not.
- Styling slotted children
- There are many limitations, but it can still be handy.
- Selecting the child.
- Selecting the slot.
- Maybe refer people to "Interacting with slotted children" for more complicated stuff, like possibly setting a class on a slotted child.

The issue is an outline for a Shadow DOM styling tutorial, covering :host, conditional host styling, reflected properties, classes and attributes, and slotted children. Start by locating the intro tutorial and the existing “Interacting with slotted children” material; done means the listed topics are explained with examples and the more complex slotted-child behavior is linked appropriately.

Written by the indexing model from the issue text.