Issue #3457507 by Grimreaper: Fix core 10.3 deprecations + use PHP attribute for plugin annotation

src/Element/AlphaColor.php

@@ -6,7 +6,8 @@ namespace Drupal\ui_skins\Element;
 
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Render\Element\FormElement;
+use Drupal\Core\Render\Attribute\FormElement;
+use Drupal\Core\Render\Element\FormElementBase;
 
 /**
  * Provides a color form element with alpha channel support.
@@ -20,10 +21,9 @@ use Drupal\Core\Render\Element\FormElement;
  * ];
  *
  * @endcode
- *
- * @FormElement("ui_skins_alpha_color")
  */
-class AlphaColor extends FormElement {
+#[FormElement('ui_skins_alpha_color')]
+class AlphaColor extends FormElementBase {
 
   /**
    * Length when encoding a color channel value in hexadecimal.

ui_skins.info.yml

 name: 'UI Skins'
 type: module
 description: 'Allow handle some theme settings like CSS variables.'
-core_version_requirement: ^10 || ^11
+core_version_requirement: ^10.3 || ^11
 package: 'User interface'