Вы можете установить новые значения в UID по умолчанию:
UIDefaults def = UIManager.getLookAndFeelDefaults();
def.put( "TabbedPane.foreground", Color.RED );
def.put( "TabbedPane.textIconGap", new Integer(16) );
def.put( "TabbedPane.background", Color.BLUE );
def.put( "TabbedPane.tabInsets", new Insets(10,10,10,10) );
def.put( "TabbedPane.selectedTabPadInsets", new Insets(10,20,10,20) );
вот список ключей
TabbedPane.textIconGap
TabbedPane.contentOpaque
TabbedPane.focus
TabbedPane.foreground
TabbedPane.tabRunOverlay
TabbedPane.shadow
TabbedPane.darkShadow
TabbedPane.background
TabbedPane.ancestorInputMap
TabbedPane.focusInputMap
TabbedPane.tabInsets
TabbedPane.light
TabbedPane.contentBorderInsets
TabbedPane.tabsOverlapBorder
TabbedPane.tabsOpaque
TabbedPane.tabAreaInsets
TabbedPane.highlight
TabbedPane.font
TabbedPane.selectedTabPadInsets
Для Nimbus LookAndFeel есть некоторые другие Nimbus по умолчанию нравится:
TabbedPane:TabbedPaneTabArea.contentMargins
TabbedPane:TabbedPaneTabArea[Enabled].backgroundPainter
TabbedPane:TabbedPaneTab[Selected].backgroundPainter
TabbedPane:TabbedPaneTabArea[Enabled].backgroundPainter
TabbedPane:TabbedPaneTab.contentMargins
TabbedPane.tabOverlap