[Steven Zahm]

@ Jon

If your parent categories are empty, may you could use the option grouping feature. This would make the root parent categories non-selectable and visually different.

You would have to edit the slim-plus.php file found here:
../wp-content/plugins/connections-slim-plus/

Look for this:

$atts = array(
	'default'    => self::$atts['str_select'] ,
	'select_all' => self::$atts['str_select_all'],
	'type'       => self::$atts['enable_category_multi_select'] ? 'multiselect' : 'select',
	'group'      => self::$atts['enable_category_group_by_parent'],
	'show_count' => self::$atts['show_category_count'],
	'show_empty' => self::$atts['show_empty_categories'],
	'parent_id'  => self::$atts['enable_category_by_root_parent'] ? self::$atts['category'] : array(),
	'exclude'    => self::$atts['exclude_category'],
	);

And change it to this:

$atts = array(
	'default'    => self::$atts['str_select'] ,
	'select_all' => self::$atts['str_select_all'],
	'type'       => self::$atts['enable_category_multi_select'] ? 'multiselect' : 'select',
	'group'      => self::$atts['enable_category_group_by_parent'],
	'show_count' => self::$atts['show_category_count'],
	'show_empty' => self::$atts['show_empty_categories'],
	'parent_id'  => self::$atts['enable_category_by_root_parent'] ? self::$atts['category'] : array(),
	'exclude'    => self::$atts['exclude_category'],
	'group'      => TRUE,
	);

If that is not an option, then you have to edit the class.template-parts.php file found here:
../wp-content/plugins/connections/includes/template/

Look for this function. You have to apply an inline style or maybe add a class that you can use CSS to select and style to the option value when $level is equal to 1. Adding to the inline style will probably be a little easier because there would be less code to modify.

Hope that points you in the right direction.