I am looking for a responsive HTML theme design framework that supports 2048px resolution (iPad retina display)– most frameworks stop at 1280px.
If there is no such framework, then how do I extend an existing framework (like Twitter Bootstrap) so that it supports 2048px?
As @Billy Moat said, Twitter Bootstrap supports high resolutions; so does Zurb Foundation.
As an aside, I found it odd that you mention the iPad retina. Did you know that while the retina iPad has a 2048px resolution, it has the same CSS pixel resolution as the original iPad? this article sums it up nicely.