fix(theme): change the dark theme default background color to transparent

[Runtus]

Checklist

• npm test passes
• benchmarks are included
• commit message follows commit guidelines
• documents are updated

Description of change

• change dark theme default color from '#141414' to 'transparent'.

• fix: 5.1.15: theme 'dark' applies black background color #6164

[coveralls]

Pull Request Test Coverage Report for Build 9016723977

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 86.373%

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Totals
Change from base Build 8998387105:	0.007%
Covered Lines:	10322
Relevant Lines:	11575

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💛 - Coveralls

[hustcc]

@Runtus 另外，官网主题 demo 中需要增加一行：

document.getElementById('container').style.background = '#141414';

[Runtus]

CI好像挂了，我本地检查下啥情况

[hustcc]

https://github.com/antvis/component/pull/318/files#diff-85d875b86904ac041094f85658fb11c5bc0ed6624d0967ae038b2edffd3cd522

可能是这里带来的，你本地删除截图，然后重新生成吧