theme-settings.php

<?php


/**
 * @file
 * Functions to support Olivero theme settings.
 */
use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter() for system_theme_settings.
 */
function olivero_form_system_theme_settings_alter(&$form, FormStateInterface $form_state) {
    $form['olivero_settings']['olivero_utilities'] = [
        '#type' => 'fieldset',
        '#title' => t('Olivero Utilities'),
    ];
    $form['olivero_settings']['olivero_utilities']['mobile_menu_all_widths'] = [
        '#type' => 'checkbox',
        '#title' => t('Enable mobile menu at all widths'),
        '#default_value' => theme_get_setting('mobile_menu_all_widths'),
        '#description' => t('Enables the mobile menu toggle at all widths.'),
    ];
    $form['olivero_settings']['olivero_utilities']['site_branding_bg_color'] = [
        '#type' => 'select',
        '#title' => t('Header site branding background color'),
        '#options' => [
            'default' => t('Primary Branding Color'),
            'gray' => t('Gray'),
            'white' => t('White'),
        ],
        '#default_value' => theme_get_setting('site_branding_bg_color'),
    ];
}