The ASP.NET Grid has the FolderStyle
property to easily change styles with only one line of code.
grid1.FolderStyle = "grid/styles/grand_gray";
grid1.FolderStyle = "grid/styles/premiere_blue";
Please check Multiple ASP.NET Grids on the same page
to see how it implemented.
You can edit the existing styles or create new ones.
Below are exactly the same grids with only FolderStyle property set to different styles.
Download ASP.NET Grid with all examples and styles
If you want to have more grids with different styles on the same page
see the Advanced Styles tutorial