der.hans,

   Misha here. Does the same thing in Ubuntu. I looked in .gimp but I couldn't find anything that looked like it set the theme colors. Then I went to gimp itself. Try opening gimp, pull down Edit menu, select Preferences, scroll down to Interface/Theme. Mine shows about 5 themes, defaulting to dark. I changed that to light, second from the bottom, and it seems like what I remember of old times. Of course click OK to exit, saving changes. 

Misha