+1000 I absolutely echo that comment. The new CWM method has me customising my DX50 with my preferred themes. Could not get either my head nor computer around ADB!
I simply attached my DX50 to my computer via USB of course, and copied the theme files from source folders into the appropriate folders in the zipped archive on the DX50. Then reinstalled the zip file using the CWM Recovery. All it did was add the themes that were not previously added. Worked fine with no errors. I guess you can add to the archive anything you need before installing. With Windows 7 and 8 if not before, you don't need to extract and rezip an archive to add files to an already compressed archive. You can add by simple drag and drop. You cannot however drag a file from between two archives. The source must be extracted first (at least that's what I found last night).
Good luck and thanks for all you RB gurus who've had a hand in this.
I use Windows Commander and you can copy files from one archive to another without problems. But if ADB will work again I will use it because that way you dont have to reinstall rockbox using CWM recovery.