Custom CSS seems not to be used

Hi,

In adminimal_theme.theme file there is function to create a adminimal-custom.css file to override or add custom css code without subthememing/hacking Adminimal Theme.

  $form['adminimal_custom']['custom_css'] = [
    '#type' => 'checkbox',
    '#title' => t('Use "adminimal-custom.css"'),
    '#description' => t('Include adminimal-custom.css file to override or add custom css code without subthememing/hacking Adminimal Theme.'),
    '#default_value' => theme_get_setting('custom_css'),
  ];

To enable the file : -
1. go on path admin/appearance/settings/adminimal_theme
2. check the "adminimal-custom.css" checkbox, file will be show on sites/default/files/adminimal-custom.css.

Test result :
It's useful for override or add custom css.