feat: init guidelines

[Mister-Hope]

No description provided.

[coveralls]

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[coveralls]

Pull Request Test Coverage Report for Build 9383802306

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Relevant Lines:	1582

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[coveralls]

Pull Request Test Coverage Report for Build 9384752644

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Covered Lines:	992
Relevant Lines:	1582

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Pull Request Test Coverage Report for Build 9396850614

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Relevant Lines:	1582

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Pull Request Test Coverage Report for Build 10502556700

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Change from base Build 10474691820:	0.0%
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Relevant Lines:	1706

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💛 - Coveralls

[Mister-Hope]

This PR is now ready for review.

Basically, this PR introduce theme guideline:

• A theme must declare attributes on certain component for plugins to "select them" correctly by default
• A theme must declare its color scheme with the given way, so that plugins can get darkmode status easily
• A theme must provide a set of colors and transition variables, so that plugins can use them and fit the theme palette by default.

Note: Since we already decide to introduce a new theme and rename the current one as classic (not sure about the progress @pengzhanbo), so I decide not to migrate the custom container and badge colors, as they are with the classic theme totally.