It should not be difficult to create a modern, elegant and responsive theme.
Creating a modern, elegant, and responsive theme independent of Centova Cast is not difficult at all. Integrating it with a project of this size, though? That's another story.
Is it really so complicated?
Actually yes, it's massively complicated and a huge undertaking.
Bear in mind that Centova Cast has been in development for a decade now, and was originally designed before the term 'AJAX' even existed. The UI was originally built in vanilla Javascript and HTML4. We then ported it to jQuery and XHTML using best-practices at the time. Now things are coming full-circle again and vanilla Javascript is once again what sites are trending toward, along with HTML5 and responsive layouts.
I'm more than familiar enough with the process to say with confidence exactly how difficult it is -- to adhere to modern HTML5/CSS standards, we're talking about modifying literally every HTML fragment and every bit of Javascript code in the project. Much of our UI code, including the media library and file manager (which represent many thousands of lines of Javascript code on their own) would need a massive overhaul to be compatible with the new layout. And that doesn't even begin to touch on the thousands of lines of jQuery-specific code we've created and which Centova Cast depends on, which -- if we're abandoning jQuery, per modern design conventions -- we'd need to rewrite as well.
Creating a nice RESTful architecture to sit behind it would require further major changes under-the-hood.
So yes, it's certainly possible. It's not something we're going to drop all other development for, however.
That said, we ARE working on modernizing Centova Cast, but the first step is modernizing it under-the-hood. We currently have an active development branch where we're refactoring Centova Cast to adhere to PSR standards (remember that Centova Cast was conceived back in 2005, when PHP4 was basically it) which, in the process, will dramatically improve ongoing maintainability and overall performance. That's all under-the-hood, but once that's done we'll be overhauling a number of user-facing components, including the UI. I won't go into further detail on that at this point, but suffice to say we've got a long list of plans to keep Centova Cast relevant moving forward.