math background color doesn't change

[matt]

i've just upgraded from lyx 1.3.5-xforms to lyx 1.4.3-qt and i find  
can't change the background color of equations like i'm used to.

i can go to tools->preferences; look and feel->colors and change the  
colors. however, only changes that aren't to the math backgrounds are  
successful. the .lyx/preferences file reflects changes to "mathbg"  
and it is definitely being read when i fire up lyx, but nothing changes.

does anyone know if this is a qt issue (like the annoying big  
operators one)? if anyone had a solution i would be very grateful.

cheers,

matt.