Write css for dropdown menu

We are looking for a webdeveloper who is able to modify our current Menu structure into a dropdown menu. The website where this needs to be done is https://hxb.nl
It is important that the mobiel and tablet version of the menu stays unchanged, so the modification is only applicable to the desktop version.
You will not be able to work on server level, however we'll supply the CSS files to you.
Looking forward to your offer.