For you, I wish it were that simple, unfortunately it is not. To replace the mstheme will require that the site be built from the ground up. What you'll end up doing is taking all of your static content like the header, the left nav and the bottom footer information and placing those into Include Pages.
FP Include Pages are similar in nature to SSI or Server Side Includes. You basically make one file for each of your static components; header, left nav and footer, and then insert those pages within your other pages.
I can't recall right now which book it was that I studied day in and day out for about a year before it all finally clicked. I believe it was a Microsoft Press book titled Using FrontPage, that was back then.
There are also two valuable online resources in addition to WebmasterWorld where you can learn a lot of the basics quickly. One of those is the Outfront Forums. Study some of the tutorials that are there as they are very well written and in laymens terms.
There is also quite a bit of valuable information in the archives here. In fact, this one thread alone is worth it's weight in diamonds!
Things to be aware of when using FrontPage [webmasterworld.com]