Microsoft®'s Windows® XP introduced a new look and feel in Windows® XP. In addition to the "Classic" boxes and gradients introduced in Windows® 95, XP has an additional "Windows® XP" style.
Classic:

The Default Visual Styles in XP:
Blue:

See XP at the Microsoft site:
http://www.microsoft.com/windowsxp/pro/evaluation/tours/default.asp
Go to the appearance tab on your Display Properties:

1. Open the display properties, and then the Appearance tab.
2. Change Windows® and buttons to Windows® XP Style.
3. The color scheme will change to allow you to select the schemes Blue, Olive, and
Silver.

Windows XP - The default Theme.
Windows Classic - The older look to windows 2000, which is currently the .NET server default.
More Themes Online... - This will take you to Microsoft's "Plus! Pack" web page.
Browse...
(NOTE : c:\windows is your XP system directory, and may be different depending on where XP was installed)

luna.theme is the Windows XP theme.
Here is the Luna directory:

luna.msstyles has the three styles : Blue, Olive Green, Metallic.
luna.msstyles also has three ShellStyles that are located in these three directories:

NormalColor is the ShellStyle for the Blue Visual Style.
Homestead is the ShellStyle for the Olive Green Visual Style.
Metallic is the ShellStyle for the Silver Visual Style.
When a change is made to a theme, it becomes modified. This includes the following :

This modified theme does not exists on disk. To save a modified theme, click Save As.
Notice that Save As defaults the saving of themes to "My Documents", and not "c:\windows\resources\themes".
Windows and Buttons lets you choose between different Visual Styles, including Classic.
Double-click an .msstyles File.
The appearance tab will open, and that style will be shown under Windows and Buttons :
Install Style XP
Add a .theme file
Many files on themexp.org have full-featured install programs, while others have just an .msstyles file.
The appearance tab will open, and that Visual Style will be shown under "Windows and Buttons" :

The coughdrop.mstyle from Style XP Plus was just double-clicked.