Hiya Patricia,
I don't think it would be all too hard if you want to take just a bit of time. Here's what I do when I'm going backwards like that ( in other words taking the theme out of the site rather than putting it in lol ).
1. You have the site setup already, if you're familiar enough with code then export the entire site to a new folder. Make a backup of that folder so you can always pull a page from it to start over if you mess it up. Basically your backup copy.
2. From here it's all just remove the content. Take out the text you put in, take out the pictures etc. so that you're left with only the theme elements.
NOTE: When you're removing things, one good way to make sure you know what is what afterwards and what to put where is to make comments
<!-- message in here -->
in the beginning of sections or at the end of particular lines of code so you can always be sure what you had there or what you want to go there and find it easier each time.
NOTE: Sometimes it may be necessary to add paragraph <p></p> or div <div></div> tags to the sections to space it out some. Add a space code in between the tags to give it body so it spaces vertically for you and doesn't look all weird like this:
<p> </p>
or
<div> </div>
Not sure if that's the route you're looking for to make a theme usable, but it should work for the most part. Hollar if you need more assistance with it