We are currently working on creating a “theme” functionality for WordCrusher that will accomodate that sort of edit inside the interface, but in the meantime you'll need to edit the source files and recompile the book if you want to change the default formatting. There's a video in the membership area on the WordCrusher page that gets into advanced editing topics:
Basically, go to the WORDCRUSHER_BOOK folder in My Documents, edit the HTML file as much as you want, and then drag the OPF file into Kindle Previewer to recompile – this will create a file in the same folder with the same filename as the OPF (but with a mobi extension) – you can also run the OPF file through KindleGen directly, but for most folks it's easier to just drag into Kindle Previewer.
If you are doing advanced edits, I recommend copying the contents of your WORDCRUSHER_BOOK folder to a new a new folder (don't delete the lic.wc or authorinfo.txt files from the original folder) and renaming the OPF file before starting to work on it. Any time you run WordCrusher, it will remove all of the previous documents files from the original folder so you don't want to accidentally lose your work.