Adjust PHP file to fix nav menu on Wordpress site

Our previous developer created a custom menu for our WP website: https://boardview.io

Now, we want to create drop down sub items in our nav menu, but the custom menu does not allow us to do so. Using the regular build in "drag and drop" menu wizard in WP would have been great for us, but it is overruled by the custom code in PHP, and does not respond.

Getting rid of the lines of custom code and going back to the default menu builder in WP could do the trick. Or otherwise we may have to add some code to the appropriate PHP files to allow for the creation of a drop down menu.

We are running the Themosis WP theme, which a quite uncommon theme. See: https://github.com/themosis

We think the code preventing drop down menu items should be somewhere in the 3 attached PHP files. If you can tell us which lines we should add or remove (or adjust the PHP file yourself), that would be great.

Looking forward to hearing from you.