fix issue #1992

[jimczj]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• [x ] Tests for the changes have been added (for bug fixes / features).
  • [ x] Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs have been added / updated (for bug fixes / features).

PR Type

What kind of change does this PR introduce?

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

Issue Number(s): #1992

What is the new behavior?

Description about this pull, in several words...

• Screens with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

In NexT _config.yml:

...

Does this PR introduce a breaking change?

• Yes.
• [ x] No.

[ivan-nginx]

Need to await for response from @geekrainy, he's make something with z-index in #1967.
Rainy, can u confirm this pull?

[geekrainy]

@jimczj @ivan-nginx I think only need to modify https://github.com/iissnan/hexo-theme-next/pull/1967/files#diff-76267f0502929e28ddc698efbf6753d7

z-index: $zindex-5;

changed to:

z-index: $index-3;

[ivan-nginx]

Fix by @geekrainy will added to v6.0.0 and to latest master branch here.