Sony Ericsson Theme File Format

Recently I wanted to pimp my Sony Ericsson mobile and add some nice themes (.thm files). There were plenty of them around and I just downloaded a collection of some hundred free ones. Unfortunately most came without a preview image and going through these on my mobile to find nice ones was painstaking and slow: I had to activate each of them, return to the standby mode, check out the main image, return to the theme manager, and start over again. What I needed was some kind of theme browser. I did not trust the Sony Ericsson Themes Creator application to provide browsing and did not bother installing it.

As being a hard core developer I had a look into the THM files. I STFW but could not find any resources explaining the file format. So I explored it a bit myself. Here is what I found together with some Ruby code.

Sony Ericsson Theme File (.THM) Format
The file contains several files concatenated together with some kind of header before the data.

• At 0x0000 a null-terminated string with the name of the original file. Remaining bytes are all null. This is read with name = data[ofs..ofs+0x63].strip.
• At 0x0064 several null-or-blank-terminated strings containing octal numbers. The number fields may contain blanks which are treated as separators as well. Parse them with

  numbers = data[ofs+0x64..ofs+0x100].
    scan(/[0-7]+/).
    collect { |n| n.to_i(8) }

• The only important number is the fourth (body_len = numbers[3]), which is the length of the data body in bytes. If this number is not positive then the file is corrupt.
• At 0x0101 there is most of the time the string ustar and at 0x0129 nogroup, but we do not need them.
• At 0x0200 starts the data: body = data[ofs+0x200...ofs+0x200+body_len].
• After 0x0200+body_len the space is filled with nulls until the next address continuing a zero low-byte. We just skip them until data[ofs] != 0.
• Repeat these steps until EOF.

Usually the first item in the theme file is called Theme.xml. This is an XML configuration file and looks something like

<Sony_Ericsson_theme version="4.5">
  <Background Color="0xb5f8fd"/>
  <Background_image Source="desktop.png"/>
  <Desktop Color="0xb5f8fd"/>

Sometimes it's not called in this way, but any XML file will do. The Color looks like a regular HTML Colour Code. The Source is a reference to an image stored inside the theme file. The important image types are

• Standby_image - main image.
• Desktop_image - background in the menu.
• Popup_image - background of popup.
• There are a lot more, but often these three images are reused in smaller themes.

Knowing the standby image name, it is possible to extract it to the file-system (File.open(filename,'wb') {|io| io.write body}). Then browsing these preview images with some image utility and deleting unwanted ones is a piece of cake. The whole Ruby program is here. Have fun!