docs(ConfigProvider): fix incorrect theme classnames

[lzxiaoqi]

… switching causes the theme styles to stack up in the document.

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解决#13179

这个bug存在于文档ConfigProvider全局配置的demo中，并且只有暗色主题存在这个问题，看了下源码，发现应该是在顶部切换主题的时候会默认执行增删van-theme-dark 和van-theme-light逻辑，而ConfigProvider全局配置内部引入了ConfigProvider在加载时会默认添加van-theme-light，应该算是文档的一个小bug，修复逻辑是暂时在ConfigProvider全局配置的示例文件中，如果同时存在这两种主题的话将van-theme-light属性删除，不会影响其他

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.65%. Comparing base (379f44a) to head (af0d041).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13187      +/-   ##
==========================================
- Coverage   89.67%   89.65%   -0.03%     
==========================================
  Files         257      257              
  Lines        7004     7006       +2     
  Branches     1732     1733       +1     
==========================================
  Hits         6281     6281              
- Misses        378      382       +4     
+ Partials      345      343       -2     

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[chenjiahan]

Thank you!