readme.md

Create custom Gin styles

Steps

1. Create a custom module (I'm calling mine mymodule) and place it in /modules/custom/mymodule
2. Add the mymodule.module file and place in the custom mode
3. Create the mymodule.libraries.yml file and place in the new library that points to the CSS
4. Create the CSS file within a css directory. Name it the same thing that you have in the mymodule.libraries.yml (gin-global.css)

gin-global.css

/* Custom Gin styles here. */
.my-gin-selector {
  color: red;
}

mymodule.libraries.yml

gin-global:
  css:
    component:
      css/gin-global.css: {}

mymodule.module

/**
 * Implements hook_preprocess_HOOK().
 */
function mymodule_custom_preprocess_html(&$variables, $hook) {
  $activeThemeName = \Drupal::service('theme.manager')->getActiveTheme()->getName();
  if ($activeThemeName === 'gin') {
    $variables['#attached']['library'][] = 'mymodule/gin-global';
  }
}