Template File Filter

The siteorigin_widgets_template_file_{$id_base} filter changes the template file that renders a widget, where {$id_base} is the widget's base ID. The Widgets Bundle uses tpl/default.php in the widget's folder unless the filter returns another path. The path must point to an existing file that ends in .php.

The filter can choose a template from a widget setting, as Extending Existing Widgets shows.

function mytheme_button_template_file( $filename, $instance, $widget ) {
    if ( ! empty( $instance['design']['theme'] ) && $instance['design']['theme'] == 'test' ) {
        // This option works for plugins.
        $filename = plugin_dir_path( __FILE__ ) . 'tpl/button.php';
        // For a theme, use this line instead.
        // $filename = get_stylesheet_directory() . '/tpl/button.php';
    }
    return $filename;
}
add_filter( 'siteorigin_widgets_template_file_sow-button', 'mytheme_button_template_file', 10, 3 );