3325719 - Create a custom admin theme and custom module

modules/custom/custom_module/custom_module.info.yml

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+name: custom module
+type: module
+description: The description.
+package: Custom
+core: 8.x
+core_version_requirement: ^8 || ^9

modules/custom/custom_module/custom_module.module

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+<?php
+
+/**
+ * @file
+ * Primary module hooks for custom module module.
+ *
+ * @DCG
+ * This file is no longer required in Drupal 8.
+ * @see https://www.drupal.org/node/2217931
+ */

themes/custom/gin_custom/css/gin-custom.css

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+/* Styles custom. */

themes/custom/gin_custom/gin_custom.info.yml

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+name: Gin Custom
+type: theme
+core: 8.x
+core_version_requirement: ^8 || ^9 || ^10
+base theme: gin
+description: 'Overrides for the Gin theme.'
+alt text: 'Screenshot of SubThemeGin, Drupal administration subtheme of claro.'
+package: Custom
+version: '8.x'
+experimental: true
+
+libraries:
+  - gin/gin
+  - claro/global-styling
+
+libraries-extend:
+  gin/gin_base:
+    - gin_custom/gin_custom

themes/custom/gin_custom/gin_custom.libraries.yml

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+gin_custom:
+  css:
+    base:
+      css/gin-custom.css: { }