change text color

[Thibaud Hulin <hulin thibaud wanadoo fr>]

Hello !
Thanks for creating Dia.
I use a dark them with gtk for visibility, and a black background with 
Dia. But, when I insert a text, the background of the text is yellow. As 
the text text is white or green, I can't read the text for modify. How 
can I modify the flashy background of the text, to adapt it to my them ?
Thanks very much,
Thibaud.