fix: tree switcher's background display unexpected in dark theme

[kiner-tang]

[中文版模板 / Chinese template]

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• TypeScript definition update
• Bundle size optimization
• Performance optimization
• Enhancement feature
• Internationalization
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Workflow
• Other (about what?)

🔗 Related issue link

• close Tree showLine icon backgroundColor #39826

💡 Background and solution

set switcher' background 'transparent'

📝 Changelog

Language	Changelog
🇺🇸 English	fix tree switcher's background display unexpected in dark theme
🇨🇳 Chinese	修复树形控件在暗黑模式下 switcher 背景显示异常问题

☑️ Self-Check before Merge

⚠️ Please check all items below before requesting a reviewing. ⚠️

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

[github-actions]

[MadCcc]

Demo 不用加了，有 e2e 测试

[codecov]

Codecov Report

Base: 99.98% // Head: 100.00% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (56ebff0) compared to base (386588d).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #39838      +/-   ##
===========================================
+ Coverage   99.98%   100.00%   +0.01%     
===========================================
  Files         557       557              
  Lines        9621      9621              
  Branches     2722      2722              
===========================================
+ Hits         9620      9621       +1     
+ Misses          1         0       -1     

Impacted Files	Coverage Δ
components/tooltip/index.tsx	100.00% <0.00%> (+1.03%)	⬆️

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