Use 7-Zip (7ZFM.exe) to open "*.thm" file and just right-click on the "theme.xml" and select "edit" (F4), it will open in notepad or your system's xml editor then just save the xml file and exit editor. 7-Zip will ask you to update the file inside archive... just click "OK" and you are done...
Your theme file is now updated with your customized "Theme.xml"...
Enjoy :)