The ".kbfxtheme" file is a simple archive file (.tar.gz or .tar.bz2) that should contain at least these elements:
One or more theme folders with all the necessary images/files inside. Have in mind that the name of the theme is the name of the folder.
In the theme(s) folder(s) there should be an image named "preview.png" that serves as a live preview of the look of the theme.
In the theme(s) folder(s) there should be a file named "version-2"
The extension of the archive should be ".kbfxtheme"
You can create ".kbfxtheme" file following these instructions (there is an option in Konqueror's rightclick "Actions" menu for preparing KBFX Theme Package OR you can create the ".kbfxtheme" file manually as described bellow):
Create a folder with a name that is identical to the name you want for your theme.
Put the necessary images/files inside it (see the above instructions for the images part).
Create an archive of the folder (with ark/tar etc).
Rename the archive file to ".kbfxtheme" and you are ready.