You need to use <ul> and <li> commands to create the dropdown.
<ul> to open the menu </ul> to close.
In the middle you open an <li> tag, add the menu names but unlike standard menu's, do not close the <li> until you get to the end of the dropdown menu item. Here is an example from one of our sites:

<ul>
<li><a href="#">TV / Audio</a>

<ul>
<li><a href="tvs/televisions.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2">Televisions</a></li>
<li><a href="audio-hifi/players-radios.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2">Audio & Hifi</a></li>
<li><a href="tvs/televisions.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#dvdplayers">DVD Recorders</a></li>
<li><a href="audio-hifi/players-radios.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#homecinemas">Home Cinemas</a></li>
<li><a href="tvs/televisions.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#stands">TV Stands</a></li>
</ul>

</li>
</ul>

<ul>
<li><a href="#">Home Appliances</a>
<ul>
<li><a href="washers-dryers/washerdryer.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#washingmachines">Washing Machines</a></li>
<li><a href="washers-dryers/washerdryer.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#tumbledryers">Tumble Dryers</a></li>

<li><a href="washers-dryers/washerdryer.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#dishwashers">Dishwashers</a></li>
<li><a href="fridges-freezers/fridgefreezer.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#fridges">Fridges</a></li>
<li><a href="fridges-freezers/fridgefreezer.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#freezers">Freezers</a></li>
<li><a href="fridges-freezers/fridgefreezer.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#fridgefreezers">Fridge-Freezers</a></li>
<li><a href="vacuum-cleaners/vacuums.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2">Vacuum Cleaners</a></li>
<li><a href="cookers-microwaves/ovens.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2">Cookers</a></li>

<li><a href="cookers-microwaves/ovens.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2#microwaves">Microwaves</a></li>

</ul>
</li>
</ul>
You can obviously attach a class to the menu with the CSS having your background so that it is styled as you would wish.

Hope that helps.