(POC, WIP) Extract Greenwich as an extension. Draft bootstrap.css variant.

.gitignore

@@ -3,8 +3,11 @@
 .use-civicrm-setup
 /ext/*
 !/ext/sequentialcreditnotes
+!/ext/csslib
 !/ext/flexmailer
 !/ext/eventcart
+!/ext/greenwich
+/ext/greenwich/extern
 !/ext/search
 backdrop/
 bower_components

ext/csslib/Civi/Csslib/Options.php

@@ -0,0 +1,44 @@
+<?php
+/*
+ +--------------------------------------------------------------------+
+ | Copyright CiviCRM LLC. All rights reserved.                        |
+ |                                                                    |
+ | This work is published under the GNU AGPLv3 license with some      |
+ | permitted exceptions and without any warranty. For full license    |
+ | and copyright information, see https://civicrm.org/licensing       |
+ +--------------------------------------------------------------------+
+ */
+namespace Civi\Csslib;
+
+class Options {
+
+  /**
+   * Options list for "csslib_autoprefixer".
+   *
+   * @return array
+   */
+  public static function autoprefixers() {
+    return [
+      'none' => ts('None'),
+
+      // PRO: Built-in, no extra dependencies.
+      'php-autoprefixer' => ts('PHP Autoprefixer'),
+
+      // PRO: Widely used, more actively maintained. Seems to update sourcemap while filtering.
+      'autoprefixer-cli' => ts('Node Autoprefixer'),
+    ];
+  }
+
+  /**
+   * Options list for "csslib_srcmap".
+   *
+   * @return array
+   */
+  public static function srcmaps() {
+    return [
+      'none' => ts('None'),
+      'inline' => ts('Inline'),
+    ];
+  }
+
+}

ext/csslib/Civi/Csslib/ScssCompiler.php

@@ -0,0 +1,66 @@
+<?php
+/*
+ +--------------------------------------------------------------------+
+ | Copyright CiviCRM LLC. All rights reserved.                        |
+ |                                                                    |
+ | This work is published under the GNU AGPLv3 license with some      |
+ | permitted exceptions and without any warranty. For full license    |
+ | and copyright information, see https://civicrm.org/licensing       |
+ +--------------------------------------------------------------------+
+ */
+namespace Civi\Csslib;
+
+use Padaliyajay\PHPAutoprefixer\Autoprefixer;
+use ScssPhp\ScssPhp\Compiler;
+use Symfony\Component\Process\Process;
+
+class ScssCompiler {
+
+  public function compile($content, $includeDirs = []) {
+    $scss = new Compiler();
+    foreach ($includeDirs as $includeDir) {
+      $scss->addImportPath($includeDir);
+    }
+
+    switch (\Civi::settings()->get('csslib_srcmap')) {
+      case 'inline':
+        $scss->setSourceMap(Compiler::SOURCE_MAP_INLINE);
+        break;
+
+      case 'none':
+      default:
+        // Nothing needed.
+        break;
+    }
+
+    $content = $scss->compile($content);
+
+    switch (\Civi::settings()->get('csslib_autoprefixer')) {
+      case 'php-autoprefixer':
+        $autoprefixer = new Autoprefixer($content);
+        $content = $autoprefixer->compile();
+        break;
+
+      case 'autoprefixer-cli':
+        $p = new Process('autoprefixer-cli');
+        $p->setInput($content);
+        $p->setTimeout(120);
+        $p->run();
+        if ($p->isSuccessful()) {
+          $content = $p->getOutput();
+        }
+        else {
+          throw new \CRM_Core_Exception("Failed to invoke the NodeJS autoprefixer via CLI.");
+        }
+        break;
+
+      case 'none':
+      default:
+        // Nothing needed.
+        break;
+    }
+
+    return $content;
+  }
+
+}