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chore: load token CSS variables within Storybook preview frame

Elwyn Benson requested to merge 2330-load-token-variables-within-storybook-preview into main

What does this MR do?

Loads the generated tokens.css (or tokens.dark.css) into the Storybook preview frame. This means these CSS variables are available for components to use for their styling.

The way these generated CSS files are scoped has been tweaked slightly, so that dark mode variables are scoped with the gl-dark class:

:root.gl-dark {
  --red-950: #fff4f3;
  ...

This means both sets of tokens can be loaded onto the page at the same time, and when running in dark mode the more specific ones will be applied.

For this to work the .gl-dark class has moved from the body element to the head element within Storybook.

This generation change also opens up the opportunity to remove all the duplicated colour values from with GitLab, and instead import + directly use these generated token variables.

Screenshots or screen recordings

(there should be no visual differences to components)

before after
image (CSS variables coming from bootstrap/_root.scss, lots of missing variables, no inverted values for dark mode) image (CSS variables coming from our generated tokens.css and tokens.dark.css)

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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Conformity

  • Code review guidelines.
  • GitLab UI's contributing guidelines.
  • If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
  • If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
  • If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
  • Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • Label as security and @ mention @gitlab-com/gl-security/appsec
  • Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

  • All actions and functionality can be done with a keyboard.
  • Links, buttons, and controls have a visible focus state.
  • All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
  • Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
  • Color combinations have sufficient contrast.

Closes #2330 (closed)

Edited by Elwyn Benson

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