fix: support dark mode for note tags in modern style

[sghuang19]

PR Checklist

• The changes have been tested (for bug fixes / features).
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Improvement.
• Code style update (e.g. formatting, linting).
• Refactoring (no changes to functionality and APIs).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

No dark color maps are specified for notes in modern style, which looks bad. See #513

What is the new behavior?

• $notes-modern-{bg,text,border,hover}-dark are added
• I simply exchanged bg with text, hover with border, and did some adjustments in lightness and saturation. Which is not perfect but acceptable in my opinion. Plus the code looks kinda elegant?

How to use?

In NexT _config.yml:

note:
  style: modern

[welcome]

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All committers have signed the CLA.

[coveralls]

Pull Request Test Coverage Report for Build 10398540209

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.446%

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Totals
Change from base Build 9517060757:	0.0%
Covered Lines:	400
Relevant Lines:	405

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💛 - Coveralls

[stevenjoezhang]

Thank you for your contribution. The previous color choices for the dark mode theme were not very good (I am not a professional designer), and the color scheme you revised is very nice. I will further test and merge the pull request.

[welcome]

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