When we have a colored background on os x we have an issue with the background of a combo box.
We either set the background to the same color or transparent but have a colored field background or we set it to white or default but have the ugly border around the rounded edged.
The solution is quite simple.
1. make the combo transparent
1. define a label with a white background
2. make the width of the combo - 5 pixels
3. make the height of the combo - 2 pixels
4. place the label behind the combo 1 pixel to the right and 1 pixel lower
et voila... All looks fine now...
Just my little 0.5 cent contribution...