update(doc): fix themeColor in configuration.

[Koooooo-7]

Summary

Fix the themeColor config doc changed in #2114 .

> <style>
>   :root {
>     --theme-color: deeppink;
>   }
> </style>

the markdown takes a mistake to treat it as HTML style. See Pink Configuration preview as reproduct.

And, it seems also brings a new feature that we could allow user set theme-color per markdown page . 🚀

What kind of change does this PR introduce?

Docs

For any code change,

• Related documentation has been updated if needed
• Related tests have been updated or tests have been added

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

Related issue, if any:

Tested in the following browsers:

• Chrome
• Firefox
• Safari
• Edge

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Name	Status	Preview	Updated (UTC)
docsify-preview	✅ Ready (Inspect)	Visit Preview	Aug 3, 2023 0:16am

[trusktr]

Oops, I forgot the code fence. I meant for this to be inside the warning. For example it should be like this:

> **Warning** Deprecated. Use the CSS var `--theme-color` in your `<style>` sheet. Example:
>
> ```html
> <style>
>   :root {
>     --theme-color: deeppink;
>   }
> </style>
> ```

and here's what it looks like inside the quote block:

Warning Deprecated. Use the CSS var --theme-color in your <style> sheet. Example:

<style>
  :root {
    --theme-color: deeppink;
  }
</style>