[#64109] Minify stylesheets in classic themes

[rutviksavsani]

Summary

Implements automated CSS minification for 11 classic WordPress themes (Twenty Ten through Twenty Twenty-One) to improve frontend performance. Minified CSS files are conditionally loaded based on the SCRIPT_DEBUG constant.

Initial reference PR: #10081

Changes Made

Build Infrastructure

• Added cssmin:themes Grunt task for automated CSS minification
• Updated .gitignore to exclude generated .min.css files from version control

Per-Theme Changes

Each theme received:

• package.json - npm build configuration with cssnano dependencies
• package-lock.json - Locked dependency versions
• contributing.txt - Developer documentation for CSS minification workflow
• Updated functions.php - Conditional loading logic using SCRIPT_DEBUG
• Updated CSS files - Added minification notice comments

Trac ticket: https://core.trac.wordpress.org/ticket/64109

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[github-actions]

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