fix: background color issue

[ruibaby]

修复使用 GitHub 风格的文章样式时，文章内容背景颜色在暗色下的样式问题。

/kind bug

before:

after:

None

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[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang, LIlGG

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/halo-dev/theme-earth/blob/main/OWNERS)~~ [JohnNiang] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang, LIlGG

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/halo-dev/theme-earth/blob/main/OWNERS)~~ [JohnNiang] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment