Finalize custom theme module contract

[BRKalow]

Currently, themes are expected to expose a default export and a named Page export. The default export is the Layout, and the named export is used as the page component for the dynamic swingset route. It might make more sense to have both of these be named exports for clarity.

// layout.ts
import { layout } from 'swingset/theme'

export default layout

// page.ts
import { page } from 'swingset/theme'

export default page