feat(plugin-markdown-chart): remove mermaid theme default variables

[pengzhanbo]

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Provide a description in this PR that addresses what the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. close #123).

What is the purpose of this pull request?

• Bug fix
• New feature
• Other

Description

I believe there is no need to set built-in theme variables in Mermaid, as the default styles are already quite excellent.

These built-in theme settings hinder users from customizing their styles.
Therefore, I think it is appropriate to remove them.

Using theme:dark directly in dark mode is more reasonable.

[coveralls]

Pull Request Test Coverage Report for Build 15698071657

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.616%

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Totals
Change from base Build 15629644930:	0.0%
Covered Lines:	1420
Relevant Lines:	2092

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💛 - Coveralls