Add darkmode of DisqusJS

[ljcbaby]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

Issue resolved: #141

What is the new behavior?

• Link to demo site with this changes: None
• Screenshots with this changes:
  

[coveralls]

Pull Request Test Coverage Report for Build 1520242562

• 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 94.468%

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Totals
Change from base Build 1411888750:	0.0%
Covered Lines:	138
Relevant Lines:	141

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💛 - Coveralls

[stevenjoezhang]

I personally recommend adapting the dark mode in the source code of DisqusJS itself, instead of NexT. This can benefit other developers and users who use disqusjs, even if they use other themes or frameworks.

[ljcbaby]

The native dark mode is mostly implemented in the color of the inherited theme, however SukkaW wraps all elements in order not to pollute.

SukkaW/DisqusJS#40 (comment)

This Pull Request is an exact copy of SukkaW's blog, with the color names adjusted.

[ljcbaby]

It looks like skk doesn't want to build in a dark mode, otherwise it would have done that already. But I also agree that a dark mode should be built in or a basic template should be given for adaptation.

[stevenjoezhang]

The indentation seems incorrect here, should be 2 spaces

[ljcbaby]

fixed