You can enable flash themes and MP3 ringtones by creating and copying "customize_upgrade.xml" to the phone using XS++ or FAR manager, without touching ANYTHING ELSE on the firmware. After you have done this, should you need to send the phone back for service, it is virtually impossible to tell you made these changes unless someone spends 10 minutes to check for it.
You can find info on flash themes on the net easily.
To enable MP3 ringtone, the code is:
<?xml version="1.0" encoding="utf-8"?>
<customization xmlns:scs="http://scs.sonyericsson.net/xsd/CDAGInput.xsd">
<drm>
<settings>
<enable-ringtoneplayer-mp3>true</enable-ringtoneplayer-mp3>
</settings>
</drm>
</customization>