Below are all the UIManager values that seem to affect the area you are targeting. As my previous answer didn't work for you I have listed all the ones I believe are relevant, I would suggest copying the below list and then commenting various ones out to monitoring the effects and see if one, or a combination will prove successful for you. I recommend putting them in before calling the tabbedpane constructor.
UIManager.put("TabbedPane.borderHightlightColor", java.awt.Color.CYAN);
UIManager.put("TabbedPane.darkShadow", java.awt.Color.CYAN);
UIManager.put("TabbedPane.light", java.awt.Color.CYAN);
UIManager.put("TabbedPane.selectHighlight", java.awt.Color.CYAN);
UIManager.put("TabbedPane.darkShadow", java.awt.Color.CYAN);
UIManager.put("TabbedPane.focus", java.awt.Color.CYAN);
I have put the color CYAN because it is very bright, and so very clear what has changed with each one.
Good luck!