The skin system is still in BETA. The window itself can be skinned but no gadgets can be at this time. You can add icons to certain gadgets by specifying an image handle or filename as the Icon$ parameter.
Check the following links for more:
http://www.f-software.co.uk/index.php?p=support&id=fui3d&help=commandref&func=FUI_Button
http://www.f-software.co.uk/index.php?p=support&id=fui3d&help=commandref&func=FUI_ComboBoxItem
http://www.f-software.co.uk/index.php?p=support&id=fui3d&help=commandref&func=FUI_ListBoxItem
http://www.f-software.co.uk/index.php?p=support&id=fui3d&help=commandref&func=FUI_MenuItem
Check the following links for more:
http://www.f-software.co.uk/index.php?p=support&id=fui3d&help=commandref&func=FUI_Button
http://www.f-software.co.uk/index.php?p=support&id=fui3d&help=commandref&func=FUI_ComboBoxItem
http://www.f-software.co.uk/index.php?p=support&id=fui3d&help=commandref&func=FUI_ListBoxItem
http://www.f-software.co.uk/index.php?p=support&id=fui3d&help=commandref&func=FUI_MenuItem