fix(theme-default): code block line number misalignment in Safari

[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?

• [x] Bug fix
• [ ] New feature
• [ ] Other

Description

The height of .line-number should be calculated based on the font-size and line-height of .line-numbers, rather than being directly set to {line-height}em, as this could lead to rendering issues.

 .line-number {
   ...
-  height: #{$line-height}em;
 }

[coveralls]

Pull Request Test Coverage Report for Build 9280098507

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 56.469%

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Totals
Change from base Build 9273639561:	0.0%
Covered Lines:	994
Relevant Lines:	1574

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💛 - Coveralls