Remove all hardcoded font sizes

[stevenjoezhang]

Two steps

1. Remove all hardcoded font sizes
2. Use em instead of px

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• 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

What kind of change does this PR introduce?

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

What is the current behavior?

Issue resolved: N/A

What is the new behavior?

• Screenshots with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

In NexT _config.yml:

...

Does this PR introduce a breaking change?

• Yes.
• No.

[1v9]

Must be the last PR for font-refacctoring 😂

[stevenjoezhang]

The first one was opened four months ago: #689

[stevenjoezhang]

Theoretically, this PR does not bring significant visual changes

[1v9]

post-title h2 这个把我搞晕了，字体太小了

[stevenjoezhang]

除了Gemini，其它scheme的post-title都设计的比较小