You can change the borderWidth property and borderColor property for the GlobalThemes collection. This changes the width and the color of the channel borders respectively for a theme. Users can then select the theme from the Themes page.
Log in to the Portal Server management console and select Portals, portal-URI, Enterprise Sample (from the Select DN pull-down menu), and Manage Channels and Containers.
Select DP XML Tree in the View drop-down menu.
Select DP_Root, GlobalThemes, and SunTheme.
Modify the borderWidth and borderColor settings and save.