ok,lets say thad you have upload "Active_desktop_black.swf" file in phone
First download TUGZip and install it.
Tar Compiler
http://http://www.tugzip.com/
XML Editor
http://www.philo.de/xmledit/
1.first right click on theme and click "Add to archive",then open the new theme.rar with winrar and rename to xxxx.tar

2.Now double click on xxxxxx.tar and it will open the archive.You will now see a number of image files and a theme.xml file.Extract all the contents of the archive to a folder on your computer, and then open the theme.xml file with the xml editor that you downloaded and installed above.

3.right after the <Sony_Ericsson_theme version="4.1">write this
<Desktop_style Type="File" Source="Active_desktop_black.swf"></Desktop_style>

4.now save the file

5.# Now you need to convert all the files back to a .thm file.
# Locate the folder on your computer where you extracted the original archive, highlight all the files and right click. You should see a green arrow that says 'add to' ON ITS OWN! Click on it.

Ok where it says format, select TAR from the drop down.
6.In the field labelled 'name' you should see the location as to where the Tar archive will be saved. Makes sure you know the location, and name it to what you what you want the theme to be called. i.e. xxxxxx.tar.Click on ok

.
7.Now rename xxxxxx.tar to xxxxxx.thm with winrar and upload it to your phone.