To do that you'd have to replace ComboBoxUI in your Look and Feel. Not everything is controlled by colors in UIDefaults. *UI classes do actual drawing.
If you are talking about the light blue border surrounding the whole combobox(button and text). You can achieve this by setting the property "Combobox. Border" in the UIManager to either null or another border.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.