Adding subtitles under navigation

I am using the theme “bluish” and wanting to add subtitles under each page title in the main menu. I have installed the subtitle plug-in, and can manage to show the current page’s subtitle only. I could put the navigation in manually in header.php, but then I am unsure how to use the “current_page_item” class.

This is an example of what I want the navigation to look like: https://themeforest.net/item/gold-wordress-blog-portfolio-cms/full_screen_preview/36292