Issue #183: Allow inclusion of theme css for paragraphs preview.

By @herbdool.

paragraphs.field_widget.inc

@@ -161,6 +161,11 @@ function paragraphs_field_multiple_value_form($field, $instance, $langcode, $ite
   }
   $field_elements['#attached']['js'][] = backdrop_get_path('module', 'paragraphs') . '/js/paragraphs_actions.js';
 
+  // Attach any paragraph specific stylesheets from the default theme.
+  if ($css = _paragraphs_theme_css()) {
+    $field_elements['#attached']['css'] = $css;
+  }
+
   return $field_elements;
 }
 

paragraphs.module

@@ -1965,3 +1965,41 @@ function paragraphs_autoload_info() {
     'paragraphs_handler_relationship' => 'views/paragraphs_handler_relationship.inc',
   );
 }
+
+/**
+ * Retrieves the default theme's Paragraphs stylesheets defined in the .info file.
+ *
+ * Themes may specify CSS files for use with Paragraphs by
+ * including a "paragraphs_stylesheets" key in the theme .info file.
+ *
+ * @code
+ * paragraphs_stylesheets[] = css/paragraphs.css
+ * @endcode
+ *
+ * @param string $theme
+ *   The theme name from which the "paragraphs_stylesheets" property should be
+ *   read in the .info files. This theme and all its parent themes will be
+ *   checked. Defaults to the current front-end theme.
+ *
+ * @return array
+ *   An array of all CSS to be added by the theme within the Paragraphs.
+ */
+function _paragraphs_theme_css($theme = NULL) {
+  $css = array();
+  if (!isset($theme)) {
+    $theme = config_get('system.core', 'theme_default');
+  }
+  if ($theme_path = backdrop_get_path('theme', $theme)) {
+    $info = system_get_info('theme', $theme);
+    if (isset($info['paragraphs_stylesheets'])) {
+      $css = $info['paragraphs_stylesheets'];
+      foreach ($css as $key => $path) {
+        $css[$key] = $theme_path . '/' . $path;
+      }
+    }
+    if (isset($info['base theme'])) {
+      $css = array_merge($css, _paragraphs_theme_css($info['base theme']));
+    }
+  }
+  return $css;
+}