theme-settings.php

<?php

/**
 * @file
 * Theme settings of the Gent Base theme.
 */

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_system_theme_settings_alter().
 */
function gent_base_form_system_theme_settings_alter(&$form, FormStateInterface $form_state, $form_id = NULL) {
  // Work-around for a core bug affecting admin themes. See issue #943212.
  if (isset($form_id)) {
    return;
  }

  $form['gent_base'] = [
    '#type' => 'details',
    '#title' => t("Gent Base settings"),
    '#description' => t("All settings defined by the Gent Base styleguide."),
    '#open' => TRUE,
  ];

  $form['gent_base']['color_scheme'] = [
    '#type' => 'textfield',
    '#title' => t("Style guide color scheme class"),
    '#default_value' => theme_get_setting('color_scheme'),
    '#field_prefix' => 'cs--',
    '#description' => t("Fill in the color scheme class to be used in your project, leave empty for the default color scheme."),
  ];

  $form['gent_base']['color_scheme_hex'] = [
    '#type' => 'textfield',
    '#title' => t("Style guide color scheme primary HEX color"),
    '#default_value' => theme_get_setting('color_scheme_hex'),
    '#field_prefix' => '#',
    '#description' => t("Fill in the color scheme primary HEX color to be used in your project, leave empty for the default color scheme. This setting is only used for displaying the favicon correctly on Android + Windows phones and Apple's Touch Bar enabled computers."),
  ];
}