[WIP-03] [Project Solar / Phase 1 / Themed CSS for HDS components] Rollup configuration and Sass processing for multiple files

[didoo]

📌 Summary

This is built on top of #3239 and is based on the initial spike done by @alex-ju in #3235

🛠️ Detailed description

In this PR I have:

• re-organized the components' Sass files architecture to split the common part in a separate file, and add two new Sass files that include theming via CSS selectors or via prefers-color-scheme
• update the Rollup configuration to process multiple Sass files and generate distinct CSS files in output:
  • design-system-components.css - current file for HDS components
  • design-system-components-theming-with-css-selectors.css - new file for theming via CSS selectors
  • design-system-components-theming-with-prefers-color-scheme.css - new file for theming via prefers-color-scheme

Then, after discussion and agreement with @alex-ju, I have moved also the existing compilation of the design-system-components.scss and design-system-power-select-overrides.scss files to our custom plugin, so we can remove rollup-plugin-scss as dev-dependency

• in the new configuration, the design-system-power-select-overrides.scss is not only copied in the dist folder, but also compiled to .css (and a .map file is generated)

⚠️ TODO

• create a Jira ticket for the future, to look at the code of rollup-plugin-scss and see if there is something else that we want to port from their implementation into our custom plugin logic
• discuss if it would make sense to have a "normal" CSS compiled for the "old" components, as is now, made of the "standard" non-themed files, and instead of compiling three different compiled "themed" CSS file, have only one for the components Scss code (it doesn't need the tokens CSS to compile!) and consumers import the "themed" CSS file they want, depending on which type of theming implementation they want to use

🔎 How to review

You can look at the single commits, to see the sequence of steps and understand the explorations and thinking behind the PR, or you can look directly at the files, checking how the Scss files have been organized, how the Rollup plugin function has been implemented, and how the path for the include declarations have been updated to make it more obvious where a file was coming from.

🛠️ How to test

Check out the branch, delete the dist folder under packages/components, run the command pnpm build or pnpm start, and see how different files are copied/generated under packages/components/dist/styles/@hashicorp (source Scss files, compiled CSS files, map files)

🔗 External links

Jira ticket: https://hashicorp.atlassian.net/browse/HDS-5505

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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 9, 2025 6:31pm
hds-website	Ready	Preview	Oct 9, 2025 6:31pm

[Copilot]

Pull Request Overview

This PR reorganizes the Sass architecture for the Helios Design System components to support theming capabilities. It creates a foundation for generating multiple CSS builds with different theming strategies while maintaining the existing component functionality.

Key changes:

• Reorganizes Sass files to separate common components from theming-specific configurations
• Updates Rollup configuration to support multiple CSS output files with custom SCSS compilation
• Creates new themed CSS variants using CSS selectors and prefers-color-scheme approaches

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/components/src/styles/components/index.scss	New centralized index file importing all component SCSS files
packages/components/src/styles/@hashicorp/design-system-components.scss	Refactored to use new components index and updated token paths
packages/components/src/styles/@hashicorp/design-system-components-theming-with-prefers-color-scheme.scss	New themed variant using prefers-color-scheme media queries
packages/components/src/styles/@hashicorp/design-system-components-theming-with-css-selectors.scss	New themed variant using CSS selectors
packages/components/rollup.config.mjs	Updated with custom SCSS compilation function and multiple output configurations