fix #20876 on master: card actions font-size not controlled by theme variable

[morenyang]

Fix card component action font-size not controlled by theme variable.

Refs to: #21106 (comment)

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

close #20876
ref: #21106

💡 Background and solution

font-size of action button in the Card component is hard-code in the stylesheet. Just change it to theme variable.

📝 Changelog

Language	Changelog
🇺🇸 English	Update font-size to theme variable in the stylesheet of the action button in card component.
🇨🇳 Chinese

☑️ Self Check before Merge

• 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 #21107 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21107   +/-   ##
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  Coverage   97.48%   97.48%           
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  Files         296      296           
  Lines        6911     6911           
  Branches     1871     1918   +47     
=======================================
  Hits         6737     6737           
  Misses        174      174

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