enhance(embed): add ability to pin color mode

[mozzius]

Supercedes #7074

• Improves styles
• Changes default theme to light

Original PR description

What

• You can now enforce the color mode to dark or light at the embed code author choices (of course you can choose system too).
  • Added an interface to the landing page that allows users to select a color mode.
  • data-bluesky-color-mode attribute is added for embed code (optional, default is system)

Why

Fix #7063

Additional Info

[github-actions]

Old size	New size	Diff
6.86 MB	6.86 MB	0 B (0.00%)

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[gaearon]

Does the in-app embed URL generator code need updating?

[mozzius]

Yes, but not urgently. It's fine now that it defaults to light mode, so basically nothing will change there. We should put it on the release tracker for next time though