The default menubar looks quite bad, I used half day to google and skin it. Ok, the result looks not too bad.

• Take out top menu item background.
• Add filter to top item label.
• Change top item rollover background.
• Set minimum width for top item.
• Set minimum width for drop down menu.
• Skinning for dropdown menu in CSS.

Full example source code download here.

File list:

• MenuBarSkin.mxml, with menu style and menuBar style especially fillColors, fillAlphas.
• AppMenuBar.as, extend MenuBar class, getMenuAt() to set minimum width for drop-down menu, updateBackground() was to remove the background.
• AppMenuItem.as, extend MenuBarItem, set top item label dropshadow filter and set minimum width for top item.
• MenuItemSkin.as, replace default up/down/over halo skin.