feat: 主题中增加图例样式的配置，便于用户了解怎么修改

[visiky]

Checklist

• npm test passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Description of change

legend 的状态样式写死在 component 中，其实是可以修改的；参考下面两个文件：

1. https://github.com/antvis/component/blob/7d74c4753ce4cc5bcb54f8f278f9ea62d6a0c6bf/src/util/theme.ts
2. https://github.com/antvis/component/blob/912fb582894be39307b508edbd0468ea7fca16d1/src/legend/category.ts#L109

closed: #3375

[coveralls]

Pull Request Test Coverage Report for Build 750911505

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• 0 of 0 changed or added relevant lines in 0 files are covered.
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• Overall coverage remained the same at 93.509%

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Totals
Change from base Build 743742278:	0.0%
Covered Lines:	10916
Relevant Lines:	11321

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💛 - Coveralls

[3012852661]

你好，我想问一下，图例的每一项内容太长超过之后会隐藏，那么鼠标hover上去的tooltip样式可以通过什么方式修改呢？