allow a site theme to include multiple styles

[parmentelat]

implements multiple style clauses in a theme as discussed in
jupyter-book/mystmd#2022

requires jupyter-book/mystmd#2910

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[Copilot]

Pull request overview

Note

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This PR updates the Book and Article themes to support configuring multiple custom CSS files via the style option and loads them in deterministic order.

Changes:

• Marked the style option as multiple: true in both theme templates.
• Updated both themes’ getCustomStyleSheet() to normalize style to an array, fetch all styles, and concatenate results in declared order.
• Updated common manifest link-rewriting to handle style as either a single value or an array.

Reviewed changes

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

Show a summary per file

File	Description
themes/book/template.yml	Declares style as a multi-value option and updates its description accordingly.
themes/book/app/utils/loaders.server.ts	Loads and concatenates multiple custom stylesheets.
themes/article/template.yml	Declares style as a multi-value option and updates its description accordingly.
themes/article/app/utils/loaders.server.ts	Loads and concatenates multiple custom stylesheets.
packages/common/src/utils.ts	Extends static link rewriting to support array-valued style.

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[choldgraf]

A few process comments:

• Please use a more informative title than this. The title should describe the change you're making without the author needing to read the PR text to understand it.
• Please link to a pre-existing github issue where we've discussed this and aligned on a general path forward, or at least +1'ed the idea in general. Merging this PR should close that issue (or in some cases make it explicit the part of the issue that it complets)

[parmentelat]

apologies, this now should be better regarding your comments