How can I save two logos for a custom theme?
Created theme-settings.php in custom theme root, there created new function, it renders new fieldset and a managed_file field 

function theme_name_form_system_theme_settings_alter(array &amp;$form, FormStateInterface $form_state) {
  $form['options_settings'] = [
    '#type' =&gt; 'fieldset',
    '#title' =&gt; t('Theme Specific Settings'),
  ];

  $form['options_settings']['add_logo'] = [
    '#type' =&gt; 'managed_file',
    '#title' =&gt; t('Additional logo'),
    '#upload_location' =&gt; 'public://logo/',
    '#default_value' =&gt; theme_get_setting('add_logo'),
    '#states' =&gt; [
      'visible' =&gt; [
        ':input[name="File_type"]' =&gt; ['value' =&gt; t('Upload Your File')],
      ],
    ],
  ];
}