Theme settings in popup

Hi! How to display some fields in a popup on the theme settings page?
In theme.yaml I added a’partial’ field:

  type: partial
  path: $/../../models/../_popup.htm

Partial content:

<div class="control-popup modal fade" id="additionalSettings">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-body">
                <button type="button" class="close" data-dismiss="modal">&times;</button>
<a data-toggle="modal" href="#additionalSettings" class="btn btn-primary btn-lg">
    Open additional settings

Looks good. One thing to consider is building a form widget that opens the popup. This would give you access to AJAX handlers.