The width of the ToolWindow's border is fixed at 1 pixel. In order to increase the width of the border, add a panel to the ToolWindow and set its Dock property to System.Windows.Forms.DockStyle.Fill. Next, set the ToolWindow's DockPadding property to the desired border width and change its background color to the desired border color.
Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2