Spontaneous background colors in treeview

[Jeffrey Barish <jeff_barish earthlink net>]

After making some changes to my program (not directly related to the GUI), I
find that one treeview has a background of alternating light and dark
bands, but only for certain data sets.  I never set the background in my
code, so I expect it to be white for every data set.  The program has this
behavior only under Ubuntu.  On another platform, I do get a white
background for all data sets (with the same code).  Does anyone have a clue
what might be causing this bizarre behavior?  If I wanted to set the
background of the treeview to alternating light and dark bands, how would I
do that?  I find that modify_bg on the treeview does nothing (although
modify_text works).
-- 
Jeffrey Barish