black-background is not longer available in gThumb since the version 3.2.0:  
* always use a dark background in the image viewer
* Image viewer: use dark background by default, and more neutral color instead of black
ChangeLog for version 3.2.0: 
2003-02-03 Paolo Bacchilega <firstname.lastname@example.org>
* src/gthumb-window.c (window_new, pref_black_background_changed)
* src/fullscreen.c (fullscreen_stop, fullscreen_start): added a black_background option to always use a black background for the image viewer.
A possible way to change the background is to edit the source code and recompile.  But I'm not a so familiar with programming and unfortunately can't help with that.