Installing themes on Windows XP was relatively straightforward:

Windows XP themes were a key aspect of the operating system's user experience, allowing users to personalize their interface and make their Windows XP installation truly their own. The variety of themes available, both built-in and third-party, showcased the creativity and diversity of the Windows XP user community. While Windows XP itself may be outdated, its themes continue to inspire and influence modern UI design.

Microsoft released these themes to promote specific products or brands. They were officially signed by Microsoft (meaning they didn't require system file patching to use) but had to be downloaded separately.

Windows XP's visual identity, introduced in 2001, marked a significant shift from the "corporate gray" of earlier versions to a more colorful, rounded interface

A fan-favorite theme that uses Xbox brand colors (green, white, and gray) to mimic the console's dashboard [7]. Modern Compatibility & Transformation