Windows 8 will bring many things to the table with regards to new functionality and visual styles. While this is great for everyone with a modern machine, Microsoft does not want to leave out those with hardware that is not up to spec for the new visual cues.
A lot of information has been leaked on Twitter today, including a screenshot showing the file structure for a new visual theme 'Aero lite". The screenshot comes courtesy of @AngelWZR who states that "generally, Windows 8 [aero lite] theme looks a lot like Metro UI in some parts. It's very flat, as I can say looking to resources from it".
It shouldn't come as a major surprise that Microsoft will bring the 'Metro' look to Windows 8 as the company has been displaying that visual style on several of its interfaces. The 'Metro' look is set to become a staple of Microsoft products as they continue to build a brand around that visual style to help differentiate their products.
Since M2 has been completed and M3 builds have been produced, more users have gotten their hands on the platform. With more users having the platform, the likelihood of leaks grows exponentially which could lead to a full build leak in the near future.