No worries Catherine 
That depends on where you’re getting the HTML for the menu.
Is that a Discourse theme?
If so, you should be able to get the CSS from it if you go to the CSS tab of the theme. You’d then only need to copy the CSS to the new theme. If not, then the process is a bit different, and I can try to help if you give me more details.
If you don’t want to worry about maintaining the code for the menu, we have an easy to configure theme component that’s similar to what you’re trying to achieve here