feat(code): add a way to configure Prism theme

[Kocal]

Summary

I didn't like Prism theme okaidia and I wanted to use another one, but it was not possible because the theme was hardcoded.
I've added a Stylus variable $prismTheme (default okaida) and changed the way to include a prism theme (directly import it in code.styl, like default Vuepress theme does).

default theme:

(Don't take care of highlighted lines, it's part of #37 😅)

override theme:

I've also updated the documentation:

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

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

• Yes
• No

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

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome 78.0.3904.108
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated (I don't think there are tests for that)

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[billyyyyy3320]

Prism's default theme @import '~prismjs/themes/prism.css won't be able to configure.

[Kocal]

Ah, nice catch

[billyyyyy3320]

For now, you can use index.stly to override it, such as:

//index.stly

@import '~prismjs/themes/prism-twilight.css'

Since our code block background is always black, some theme might not be suitable.

AFAIK, VP's default theme doesn't support configuring Prism theme. I can't confirm whether it should be supported, but it's worthwhile to open an issue in VuePress, and we can follow the decision.

[Kocal]

Yes, VP's default theme does not support configuring Prism theme, they directly use @import '~prismjs/themes/prism-tomorrow.css' in styles/code.styl.

Using your solution won't work properly because Prism is still imported with <style src="..."> in this theme. It seems that Prism need to be imported with @import in Stylus in the base theme in order to be overridable, see vuejs/vuepress#1707.

I will open a new PR to remove <style> and import Prism in file code.styl, like it's done in VP's default theme.
Like this, end users will be able to override it in their index.styl.