it's possible, but it's painful to edit the files, you need editor capable of editing pvr files, and still after that, you would need to distribute edited files with your level and every one would need to overwrite existing theme file, also, that would change the level theme to your edited one => too complicated to mess with it.

if you change theme variable in the editor edit box to theme1..4 (might have more, you need to try) it will change the theme in the level.