A DESCRIPTION OF THE PROBLEM :
DefaultMetalTheme.getPrimaryControlShadow() is used for the main scrollbar handle color, combo box selection background color, and part of JInternalFrame selected border. This makes it non-trivial to subclass and create a dark version of Metal, where you would want scrollbar handles to be a subtle dark color and not a selection color. Adding DefaultMetalTheme.getMenuSelectedBackground() and DefaultMetalTheme.getMenuSelectedForeground() would be a good solution.

A work-around is to manually set the UIManager "ComboBox.selectionBackground" property for dark metal themes.