feat(theme-default): adjust the color contrast in dark mode

[yuyutaotao]

Summary

Improve the color contrast of default theme in dark mode.

According to WebAIM, the default color contrast for Rspress is low at 11, which makes the docs difficult to read. Here I changed the default theme color to improve the contrast ratio to 16.

Here i changed the default theme color to improve the contrast to 16.

Some data for reference:

Site	Contrast (by WebAIM)
NextJS	17.93
ReactJS	13.95
Rspress (before this PR)	11.65
Rspress (after this PR)	16.9

Related Issue

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Timeless0911]

LGTM

[chenjiahan]

LGTM 👍