The default theme for Windows 10 is the "aero. theme" file in the "C:\Windows\Resources\Themes" folder. Option 1 or 2 in the tutorial below can help show you how to change your theme to the default "Windows" theme if needed.
The default theme that is installed with Windows XP is called "Luna". It is a blue and green theme with a metallic finish. It was designed to be calming and easy on the eyes.