i always pick a base theme that has already been approved by the roms developer for his rom....
then through 7zip you can easily transfer any .pngs you want from same folder to same folder.
if the base theme is missing a something from the app folder that you need, first check in the current release rom and if it is available then drag/drop it using 7zip to your base theme....if it is not available then that means the dev removed it for a reason and it doesnt get ported by you, otherwise you are putting something back in the dev. took out for the end user.
once you learn where most of the .pngs are (or at least the ones you care about) and have swapped drag/drop through 7zip to your base theme, then you can test boot it.
here you will notice visibility issues (and hopefully thats it....lol)....at that point you have to change status bar colors, popup background colors, etc. (all covered in my write up that is now screwed as my over bandwidth photo whatever crap).
of course there is more to it then that as things go on, but its a good place to start for a learning experince and quick port.
so say i want black white and chrome in fission 2.5.7, but all i have is black white and chrome in gummy jar.
first pick a theme that is approved for fission 2.5.7, this is now my base theme.
next take bwc and open it and your base in 7zip...two seperate 7zips.
start swapping all pngs from same folder to same folder (select all/drag and drop are your friend here). i port everything by hand, i dont use auto porter things.
when you're done (and you never are) close it up and test boot it.
when selecting a base theme, if you have options, look at the colors in the status bar and the pup ups and see if you can quickly pick one that seems it will match the theme you are bringing in to it. this way you can avoid color changes and pop up changes for visibility.
rememver if you go editing pngs you need to know how to deal with .9's....see my totally worthless huge amount of time spent on theme hlep section....thanks photo bucket. lol...
edit: also remember if something doesnt look right, it may not be....dont kill yourself looking through every .png, unless you are looking for something specific....then you may have to, but say you open music.apk in both base and getting ported theme and they look nothing alike inside....there may be something else going on....so until you get to where you can figure it out....screw it; delet music from the base and dont theme it. (do not, do not do this inside framework....lol....or if you do let me know what happens).
^anytime....