Which Windows version introduced the 'Luna' visual style?

Windows XP introduced the Luna visual style. It replaced the comparatively restrained Windows Classic appearance with a colorful interface featuring rounded window borders, a green Start button, and brighter controls.

Luna was officially referred to as the Windows XP style and was installed by default in Windows XP Home Edition and Professional. It came in three main color schemes: the default blue, Olive Green, and Silver. The design gave Windows XP its immediately recognizable look.

The style was part of a broader interface overhaul. Windows XP added visual-style support through version 6 of the common controls library, allowing compatible applications to adopt themed buttons, scrollbars, title bars, and other controls. Users who preferred the older appearance could still switch back to Windows Classic.

Luna is often confused with Aero, the glassy interface associated with Windows Vista. Aero arrived in Vista several years later. Windows XP also had other named themes, including Royale for some Media Center editions, but Luna remained the signature visual style of the standard XP release.