imp(fonts) use font-display swap to prevent FOIT

[Kocal]

Summary

When the custom font PT Serif is downloading, headings are not visible until the custom font is downloaded.
This is what we call Flash Of Invisible Text (FOIT).

To prevent this, we cant set @font-display: swap to:

• use the fallback font when PT Serif is being downloaded
• use PT Serif when it's downloaded

This is called Flash on Unstyled Text (FOUT), and it's preferable over FOIT since you can directly see the content.

Read more at: https://css-tricks.com/font-display-masses/

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 80.0.3987.116
• 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]

LGTM!