fix(docs): remove hardcoded VitePress brand color override

[SonyLeo]

问题处理：

修复了文档站点在暗色模式下侧边栏悬浮标题时颜色不可见的问题。

具体处理方式是移除 VitePress 主题中对 --vp-c-brand-1: #191919 的硬编码覆盖，让默认的暗色模式品牌色变量重新生效。

修改前：

修改后：

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• docs/.vitepress/theme/style.css

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