Resources:your java phone only
software:bluetooth file transfer(search it in this site)
texteditor(download it at ******) and that is all
first of s40 theme consist of pics(usually .jpg,.gif,.png,.swf,etc) and theme descriptor(basically,it may contain sounds etc as well),you can easily make your pics
as for theme descriptor you need the text editor above,writing a theme descriptor is repetitive process to save time extract any theme descriptor(henceforth,denoted td) using bft above from any public theme,navigate to the folder where you have extracted the td with your text editor modify it as you deemed fit e.g change colours,pics names to correspond to names of your pics.Next go to options click save.close text editor(notepen and save it as utf,first option).Finally move your pics,td etc to a folder which contain nothing else.Open bft navi. to the aforementioned folder open it,go to options select all,next go to options compress as .nth file(accept the 2 choice).Finally you have s40 theme.It work!Try it.Hope i didn't bore you silly since it is loooong i will be posting future if any as a problem.