fix: --scroll-bar var default value

[MYKEU]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
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🔗 Related issue link

💡 Background and solution

--scroll-bar is not initially defined, thus, receiving the following when compiling:

 variable --scroll-bar is undefined and used without a fallback

📝 Changelog

Language	Changelog
🇺🇸 English	Fix variable --scroll-bar is undefined
🇨🇳 Chinese

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• 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

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[codecov]

Codecov Report

Merging #22754 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #22754   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
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  Files         360      360           
  Lines        7213     7213           
  Branches     1934     1971   +37     
=======================================
  Hits         7089     7089           
  Misses        124      124

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