feat(theme-default): improve sidebar headers

[pengzhanbo]

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

• [x] Read the Contributing Guidelines.
• [x] 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
• [x] New feature
• [ ] Other

Description

[coveralls]

Pull Request Test Coverage Report for Build 9380781281

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 56.168%

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Totals
Change from base Build 9379255829:	0.0%
Covered Lines:	992
Relevant Lines:	1582

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💛 - Coveralls