Align styles (CSS) with source.coop

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From Slack

harmonize the CSS and fonts used by the viewers and the CSS of the overall app. I'd like them to appear as seamless as possible.

• adapt CSS for the light theme
• adapt CSS for the dark theme (triggered by @media (prefers-color-scheme: dark))
• document the decision to copy/paste the styles, vs depending on a shared theme (in an ADR in https://github.com/source-cooperative/source.coop? https://github.blog/engineering/why-write-adrs/ - https://understandlegacycode.com/blog/earn-maintainers-esteem-with-adrs/#an-adr-has-5-essential-elements)

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see #35

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See https://github.com/source-cooperative/source.coop/wiki/0001-%E2%80%90-Copy-styles-manually-from-source.coop-to-embedded-viewers for the ADR.