Theming - Allow manipulation of Angular CSS

[totten]

Overview

The Theming subsystem allows themes to do things like:

• Override a CSS file (by creating an eponymous CSS file)
• Exclude a CSS file (by declaring the excludes list in hook_civicrm_themes)

This was originally implemented when all screens used QuickForm. We now also have screens implemented as Angular modules. They have declarations like:

// FILE: ang/api4Explorer.ang.php 
return [
  ...
  'css' => [
    'css/api4-explorer.css',
  ],
  ...
];

As a themer, you would expect the CSS overrides to work the same way for QuickForm pages and Angular pages.

Steps to Reproduce

• Take an existing theme
• Add a file css/api4-explorer.css. Fill it with dummy content.
• Open "APIv4 Explorer"
• View the source. Find where includes api4-explorer.css.

Before

• Overrides and exclusions for Angular CSS are quietly ignored.
• The example api4-explorer.css is loaded from civicrm-core.

After

• Overrides and exclusions for Angular CSS work.
• The example api4-explorer.css is loaded from your theme.

[civibot]

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[vingle]

This works for me. And, most !importantly, helps remove something !important, that themers are forced to use.