add multi-color scheme support

[jiangtj]

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Issue Checklist

• I am using NexT version 8.0 or later.
• I have already read the relevant documents of Hexo and NexT.
• I have already searched for current issues, which does not help me.

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Expected behavior

I found out that GitHub changed its darkmode scheme. It can choose two colors to be switched automatically by the system, or select a single color scheme. So we can refer to it to improve our theme.

Single theme

Sync with system

For our theme style, it is easy to implement, since mimi extracted the colors

for example

blue.styl

@import 'light';
// custom color ...
:root {
  --text-color: blue;
}

But for some third-party plug-ins, we need to consider how to integrate. 😑

for example

[liushulun]

Expect for that too!

Theme can only be switched by OS or browser for now. Add [Prefer theme] to force only one theme will be really helpful to me 🙏