You will neeed to obtain an existing theme and edit the file using Notepad to use that SWF flash file.

Then you can unplug your phone and the flash themes are on your phone.
But this is no good as you are not using them with anything so these tutorials will tell you how to make them work with any flash theme you have put on your phone. ( Note:I did not make this).Thanks uroszg!
Flash Tutorial
Attached File themeflash.zip ( 1.3MB ) Number of downloads: 1121

Text Tutorial
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If you are getting problems with doing this like an error message when you install them on your phone like " Theme file corrupt " or the flash menu is not there then make sure.
You have put the EXACT name of the swf so if it is " citybeat.swf " you NEED to put the whole name not just " citybeat " or " CiTyBeAt.swf " as it is CASE-SENSETIVE. Also if this does not work remember to .tar the theme files WITH TUGZIP and NOT 7-zip.Then you can rename to .thm
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Tutorial | W850i Flash Themes Guide Cid/49/51/51/53 - SE-NSE Forums