feat(theme-default): nomalize rem size

[Mister-Hope]

No description provided.

[Mister-Hope]

@meteorlxy Inspired by #816, following root font size have better a11y.

[Mister-Hope]

Then how about adding an addtion style:

: root {
  font-size: 16px;
}

or

html {
  font-size: 16px;
}

Anyway, my idea is we should not unbind font-size of main text with rem, while let the remaining site elment bind with rem.

[meteorlxy]

What about simply moving font-size: 16px; to the html, body section? What's the point to specify 1rem for body?

[Mister-Hope]

Setting both as 16 px, user will fail to change body font-size when simply setting a new rem size.