Templatestyle Form Field
The templatestyle form field type provides a dropdown list of template style options
- type (mandatory) must be templatestyle.
 - name (mandatory) is the unique name of the field.
 - label (mandatory) (translatable) is the field html label.
 - description (optional) (translatable) is the field description.
 - client (optional) site or administrator, defaults to site.
 - class (optional) for styling.
 - multiple (optional) is whether multiple items can be selected at the same time (true or false). In Joomla 4 it is recommended to use additionally 
layout="joomla.form.field.groupedlist-fancy-select"in the field declaration (replacement for the 'chosen' library used previously). 
Implemented by: libraries/src/Form/Field/TemplatestyleField.php
Example XML parameter definition
<field
        name="admin_style" 
        type="templatestyle"
        client="administrator"
        description="COM_ADMIN_USER_FIELD_BACKEND_TEMPLATE_DESC"
        label="COM_ADMIN_USER_FIELD_BACKEND_TEMPLATE_LABEL" 
        >
        <option value="">JOPTION_USE_DEFAULT</option>
</field>