refactor: prefer CSS custom properties over Shadow Parts

[rdlabo]

Summary

• move eligible component styling from Shadow Parts to documented Ionic CSS custom properties
• keep Shadow Parts only where public variables are unavailable or would affect a different internal element
• preserve existing glass appearance, interaction states, RTL behavior, and user override paths

Compatibility review

• no demo-specific IDs, routes, fixtures, or DOM ordering assumptions
• selectors use Ionic components and standard runtime state classes
• 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
• Prettier check for src
• git diff --check
• 87 Mac Chrome light/dark screenshots matched the before-change baseline; the excluded dark sheet case is an unchanged baseline flake
• verified pressed single/dual range transforms and collapsing-header button behavior

Linux Playwright snapshots were not modified.

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

[github-actions]

Playwright test results

 90 passed

Details

 90 tests across 2 suites
 1 minute, 43 seconds
 20af967
 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-ios26/pr-133/

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-ios26/pr-133/

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-ios26/pr-133/

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

[github-actions]

npm beta published

CI passed for the merge commit 466e5b66db23. Install the immutable version with:

npm install @rdlabo/ionic-theme-ios26@9.0.2-beta.pr133.sha466e5b66db23