Add Styling page about multi-tentant / theme-switching approaches

[rolfsmeds]

Theme-switching seems to still be a problem for developers, so the Styling docs could include a page that describes various approaches in detail:

• Theme variants included in bundled stylesheets, scoped with classnames
• Loading additional stylesheets dynamically (via URL)
• Loading entire stylesheet from DB, and adding it to the UI
• Loading individual style values (e.g. colors) from DB and applying them to custom properties

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