chore: Update font-sizes in AlertReportModal

[geido]

SUMMARY

This PR removes hacky usages of font-sizes by replacing them with the exact font-sizes available in the theme config.

BEFORE

AFTER

TESTING INSTRUCTIONS

1. Go to Alerts & Reports
2. Add a notification method Email
3. Make sure the font-size of the helper text looks balanced

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #19615 (7d76640) into master (83c3779) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #19615      +/-   ##
==========================================
+ Coverage   66.63%   66.66%   +0.02%     
==========================================
  Files        1680     1681       +1     
  Lines       64292    64309      +17     
  Branches     6564     6565       +1     
==========================================
+ Hits        42839    42869      +30     
+ Misses      19751    19747       -4     
+ Partials     1702     1693       -9     

Flag	Coverage Δ
javascript	51.52% <100.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...frontend/src/views/CRUD/alert/AlertReportModal.tsx	52.08% <100.00%> (-0.30%)	⬇️
...ntend/packages/superset-ui-core/src/color/utils.ts	100.00% <0.00%> (ø)
...c/views/CRUD/data/database/DatabaseModal/index.tsx	32.79% <0.00%> (ø)
...ins/legacy-plugin-chart-partition/src/Partition.js	0.00% <0.00%> (ø)
...ins/legacy-plugin-chart-sankey/src/ReactSankey.jsx	0.00% <0.00%> (ø)
...s/legacy-plugin-chart-horizon/src/HorizonChart.jsx	0.00% <0.00%> (ø)
...acy-plugin-chart-paired-t-test/src/PairedTTest.jsx	0.00% <0.00%> (ø)
...rt-parallel-coordinates/src/ParallelCoordinates.js	0.00% <0.00%> (ø)
...allel-coordinates/src/ReactParallelCoordinates.jsx	0.00% <0.00%> (ø)
... and 7 more

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[michael-s-molina]

LGTM