[WIP-04] [Project Solar / Phase 1 / Showcase] Add support for theming and theme-switching to the showcase

[didoo]

📌 Summary

This is a WIP, which contains the porting of previous showcase theming explorations (squashed).

This is based on top of #3259

🛠️ Detailed description

👉 👉 👉 Preview: theming page

💬 To discuss/decide

• do we want to add hds-theme-system or data-hds-theme="system" when the user-selected option is system?
  • pros: it's clearer what theme is applied, by looking at the HTML
  • cons: it required some work to have the selector work with the :not() selectors (see below)
• should the _currentStylesheet be stored in local storage or in URL as query param (same for the other shw preferences)?
• do we want to keep the globalOnSetTheme callback? which use cases do we foresee?

🗒️ TODOs

• check if we need to update/fix the CSS selector :root:not([class*="hds-theme-"],[data-hds-theme]) (it may not work, if we use hds-theme-system or data-hds-theme="system")
• see if there is something we can get from these:
  • https://github.com/IBM/carbon-components-ember/blob/main/docs-app/app/docs-support/theme-support.gts
  • https://github.com/IBM/carbon-components-ember/blob/main/docs-app/app/docs-support/theme-switcher.gts
  • https://github.com/universal-ember/ember-primitives/blob/main/ember-primitives/src/color-scheme.ts
• write tests for the hdsTheming service
• write tests for the HdsTheming component

📸 Screenshots

🔗 External links

Jira tickets:

• https://hashicorp.atlassian.net/browse/HDS-5518
• https://hashicorp.atlassian.net/browse/HDS-5532

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👀 Component checklist

• Percy was checked for any visual regression

💬 Please consider using conventional comments when reviewing this PR.

📋 PCI review checklist

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've worked with GRC to document the impact of any changes to security controls.
  Examples of changes to controls include access controls, encryption, logging, etc.
• If applicable, I've worked with GRC to ensure compliance due to a significant change to the in-scope PCI environment.
  Examples include changes to operating systems, ports, protocols, services, cryptography-related components, PII processing code, etc.

[vercel]

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Project	Deployment	Preview	Updated (UTC)
hds-showcase	Ready	Preview	Oct 21, 2025 1:37pm
hds-website	Ready	Preview	Oct 21, 2025 1:37pm

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