To customize appearance of Application Button in any style after Office 2007 (like 2013 etc.) you need to modify the application button color table on Office2007ColorTable like so:

c#:

Office2007ColorTable table = ((Office2007Renderer)GlobalManager.Renderer).ColorTable;
Office2007ButtonItemColorTable defaultTable =table.ApplicationButtonColors[0];
defaultTable.Default.Background=new LinearGradientColorTable(Color.Red);

VB:

Dim table As Office2007ColorTable = DirectCast(GlobalManager.Renderer, Office2007Renderer).ColorTable
Dim defaultTable As Office2007ButtonItemColorTable = table.ApplicationButtonColors(0)
defaultTable.[Default].Background = New LinearGradientColorTable(Color.Red)

If you are using Office 2003 style then follow instructions below to customize application button.

Default appearance of Ribbon Application Button is a round glossy button as shown in the image below:

To customize its appearance or to completely remove the background round part of the button, modify following properties on Office 2007 Color Table:

Office2007ColorTable.RibbonControl.StartButtonDefault
Office2007ColorTable.RibbonControl.StartButtonPressed
Office2007ColorTable.RibbonControl.StartButtonMouseOver

These properties are Image type which you can set to the custom background image you want to use on button or you can set them to null/nothing to completely remove the round background of the button.

Related posts:

  1. How To Customize Office 2007 Color Table
  2. How to Customize DateTimeInput, DoubleInput, IntegerInput and TextBoxX colors
  3. How to Access Office 2007 Color Table
  4. How to customize Office 2007 color table on per Bar or Control instance
  5. How to change & apply Ribbon Color Table