docs: fix meta tag theme color

[SID12g]

When connecting nextra docs, theme color is maintained in #fff even though it is in dark mode, so I modified it so that the theme color can be determined according to the presence or absence of the user's dark mode.

Existing docs appearance

After changing meta information

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Latest commit: eb57f7d

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Name	Status	Preview	Comments	Updated (UTC)
nextra-v2	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 9, 2024 4:29am

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Name	Status	Preview	Comments	Updated (UTC)
nextra	⬜️ Ignored (Inspect)	Visit Preview		Sep 9, 2024 4:29am

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