refactor: prefer CSS custom properties over Shadow Parts

[rdlabo]

Summary

• move eligible back-button, range, toggle, item, and detail-icon styling to documented Ionic CSS custom properties
• keep Shadow Parts only for dimensions, transforms, and borders without an equivalent public variable
• preserve existing component appearance, color-state behavior, RTL behavior, and user override paths

Compatibility review

• no demo-specific IDs, routes, fixtures, or DOM ordering assumptions
• selectors cover Ionic color and checked combinations
• user overrides remain possible through ordinary later CSS custom-property declarations
• manager review and independent acceptance review completed with no blocking findings

Validation

• npm run build
• npm run lint
• git diff --check
• 10 affected Mac Chrome light/dark screenshots matched the before-change baseline

Linux Playwright snapshots were not modified.

Important-declaration cleanup is intentionally split into a separate stacked PR.

[github-actions]

Playwright test results

 78 passed

Details

 78 tests across 1 suite
 1 minute, 36 seconds
 a64b120
 This detailed result covers Ionic 9 only. Ionic 8 runs against the same screenshots in a separate matrix job; check the workflow run for both results. To update the screenshots, comment with /update-screenshots.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-39/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-39/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-39/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.