you basically have to do this:
download a software like TUGzip for example.
Take a Theme file you already have from one of those flash themes.
Rename the extension from .THM to .TAR
Open it with TUGZip and extract the contents to a new folder
Open the .xml file in there with notepad
In line 4 or something, underneath Created by Sony Ericsson Theme Creator there the name written in Inverted commas of the file whose .THM's contents you have extracted. Rename that form for example "clubbeat.swf" to "(yourfile).swf"
Now save and close
Highlite all the files and right click. Click add to , and select .TAR. If this does not appear using TuGZip compress all these files to a .TAR extension file.
Rename .TAR to .THM and put it on ur phone, it should work.