When you use a dark theme on Xfce 4.8, the window to "Add new widgets" from xfce4-panel use the color to draw the window, but it shouldn't. Using Arch Linux x86_64 and Xfce 4.8. Tested on x86 and this bug was there too.
Created attachment 3419 Bug example
Bug in the theme, it sets special styles on Xfce*Panel* widgets and classes and that seems to hit the item dialog too...