fix: fix the post sidebar_prepend zindex

[xinkeng0]

fix #84

https://tailwindcss.com/docs/z-index#arbitrary-values

If you need to use a one-off z-index value that doesn’t make sense to include in your theme, use square brackets to generate a property on the fly using any arbitrary value.

[f2c-ci-robot[bot]]

@gengxiaoxiaoxin: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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

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,ruibaby] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment