refactor: add alert-close-hover-color

[JLHwung]

First of all, thank you for your contribution! :-)

Please makes sure that these checkboxes are checked before submitting your pull request, thank you!

• Make sure that you propose pull request to right branch: bugfix for master, feature for branch feature.
• Make sure that you follow antd's code convention.
• Run npm run lint and fix those errors before submitting in order to keep consistent code style.
• Rebase before creating a pull request to keep commit history clear.
• Add some descriptions and refer relative issues for you pull request.

Extra checklist:

if isBugFix :

• Make sure that you add at least one unit test for the bug which you had fixed.

elif isNewFeature :

• Update API docs for the component.
• Update/Add demo to demonstrate new feature.
• Update TypeScript definition for the component.
• Add unit tests for the feature.

elif isRefactor :

Introducing breaking-change?

• Yes
• No

Background information:

My company are theming ng-zorro-antd and it turns out some style declarations should be extracted to less variables for theming convenience.

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[codecov]

Codecov Report

Merging #13768 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #13768      +/-   ##
==========================================
- Coverage   93.75%   93.68%   -0.08%     
==========================================
  Files         314      314              
  Lines        6633     6633              
  Branches     1762     1765       +3     
==========================================
- Hits         6219     6214       -5     
- Misses        403      408       +5     
  Partials       11       11

Impacted Files	Coverage Δ
components/_util/wave.tsx	84.84% <0%> (-3.04%)	⬇️
components/upload/UploadList.tsx	88.49% <0%> (-1.77%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f99b8a1...bfbb406. Read the comment docs.

[codecov]

Codecov Report

Merging #13768 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #13768      +/-   ##
==========================================
- Coverage   93.75%   93.68%   -0.08%     
==========================================
  Files         314      314              
  Lines        6633     6633              
  Branches     1762     1765       +3     
==========================================
- Hits         6219     6214       -5     
- Misses        403      408       +5     
  Partials       11       11

Impacted Files	Coverage Δ
components/_util/wave.tsx	84.84% <0%> (-3.04%)	⬇️
components/upload/UploadList.tsx	88.49% <0%> (-1.77%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f99b8a1...bfbb406. Read the comment docs.

[codecov]

Codecov Report

Merging #13768 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #13768      +/-   ##
==========================================
- Coverage   93.75%   93.68%   -0.08%     
==========================================
  Files         314      314              
  Lines        6633     6633              
  Branches     1762     1765       +3     
==========================================
- Hits         6219     6214       -5     
- Misses        403      408       +5     
  Partials       11       11

Impacted Files	Coverage Δ
components/_util/wave.tsx	84.84% <0%> (-3.04%)	⬇️
components/upload/UploadList.tsx	88.49% <0%> (-1.77%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f99b8a1...bfbb406. Read the comment docs.

[codecov]

Codecov Report

Merging #13768 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #13768      +/-   ##
==========================================
- Coverage   93.75%   93.68%   -0.08%     
==========================================
  Files         314      314              
  Lines        6633     6633              
  Branches     1762     1765       +3     
==========================================
- Hits         6219     6214       -5     
- Misses        403      408       +5     
  Partials       11       11

Impacted Files	Coverage Δ
components/_util/wave.tsx	84.84% <0%> (-3.04%)	⬇️
components/upload/UploadList.tsx	88.49% <0%> (-1.77%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f99b8a1...bfbb406. Read the comment docs.