There are a couple issues here:
1) The icon for the menu button was being drawn in the backgound of the button not its icon so when a custom icon was used it was drawn inside the default icon not in stead of. I have moved where the icon graphics are drawn which fixes this.
2) If the custom icon is bigger than the default (>18x18px) then the header paints incorrectly. There is no simple way to fix this with the way the Synth Regions are setup and the way Nimbus design looks. There is a property to restrict the frame icon size, which is now restricted to 18x18px which should fix this in all common casses.
3) Notcied that the title text font should be bold to match Nimbus Spec. FIXED