Sony Ericsson mobile themes (.thm files) is simply a collection of a lot of images and an xml file that specifies the colour codes. Take a theme file rename it to .tar (use archive manager on linux or any standard compresssing utility) and unzip it using any standard unzipping utility. Just play with the files and images and you can make your self a new theme. Be it for W550i, W800i, W880i, K310i, K750i, K700i, etc ... Re-zip the contents and rename it to .thm file and you have a new theme. Copy it to the themes folder on your mobile. Set the theme !
Theme files for Sony Ericsson phones, except for the Symbian™/UIQ™ phones (P900, P910, P990, M600
and W950 series), are TAR archives with the file extension .thm. A theme file contains at least an XML file,
in which colour settings for graphical elements and optionally image file names and audio file names are
defined. If the XML file specifies image or audio file names, the specified files are also included in the
archive. To extract the individual files from THM theme files most standard archiving applications on the
market can be used.
Sony Ericsson Symbian/UIQ phone theme files are ZIP packages with file extension .utz. A package contains
an XML file specifying colour settings, images and sounds used in the theme, together with a
number of image and audio files.
Simple and Short :)
Sony Ericsson Themes Creator
If you cannot take the pain of doing the above steps, simple download the sony ericsson themes creator from here