docs: warn of styling config restriction (close #1889)

[billyyyyy3320]

Summary

Relevant issue #1889

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
• 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

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]

@meteorlxy Thanks for the review, I've just updated it.

[meteorlxy]

Maybe we should add a tip for how to import .css files?

For example: in enhanceApp.js?

[meteorlxy]

Another point, relative .css file is not supported. How about css file form npm packages? 🤔 It might work. I'm going to have a try

[meteorlxy]

I tried with github-markdown-css

Add following snippet in .vuepress/styles/index.styl:

@require '~github-markdown-css/github-markdown.css'

Things work well.

So I think we need to make the tip more specific @newsbielt703

[billyyyyy3320]

@meteorlxy Cool! And the point is only relative path is invalid, so using webpack alias to make it an absolute path is a solution. I've updated.