I'm not entirely sure what went wrong for you with your edits, they are both fairly trivial.
For the fonts, in the .sbs file you will find them defined in lines 17, 18 and 19. For the .wps you will find the fonts defined in lines 16, 17 and 18:
You should edit the .sbs and .wps by supplying your desired fonts name(s) in replacement of the font(s) you wish to replace, and replace the themes original font files with your desired fonts:
That would be the only modifications needed to replace your desired font(s). As for the backdrop, all you need to do is replace the backdrop.bmp file with one of your own, or the edited original while respecting the original file's dimensions (it must be the same dimensions as the screen resolution) and naming scheme (backdrop.bmp) and saving as a 16/24bit .bmp
Note for amayer:
There's an odd syntactical error, which is apprently harmless, but should be corrected in lines 34 and 37 of the .sbs and in lines 33 and 36 of the .wps where you use alignment tags on a viewport declaration. It seems it is only used this way for the album art viewports, perhaps in an effort to center align the album art, however album art is aligned (correctly, in the theme itself) by the optional alignment tuples in the %Cl tag as you can see in line 28 of the .sbs and line 27 of the .wps
"you can't use the alignment tags for a viewport declaration"