Using Multiple Themes

Hello

I have created the following website using a WP Template:
https://eastcoasttechwriter.ca/wordpress/

However, my customer has decided that whenever they click on
a main menu item from the top menu (example: Reimbursement),
the menu items underneath this should display in the left
column. Example url: https://www.scribesoft.com/Solution-Overview.asp

Is this possible to do this using a WP Template?
If so, how would I start to do this?

Thanks.