Twitter bootstrap is an amazing responsive css framework. It comes with default look and feel. Bootswatch has a very nice collection of free themes to override the default look and feel of Twitter Bootstrap. Cosmo is one of the bootswatch themes styled to look like windows 8.
To change the default look and feel of twitter bootstrap just replace the CSS file with that one you downloaded from the bootswatch site and enjoy the new look.
You may also have a look over the other themes in the gallery and select one that is suitable for your next project. All themes are open source an lichened under Apache 2.0.