A developer might not do as good a job at it as someone who focused their career at it, but I would be surprised if developers couldn't put together something that was at least passable if they put a modicum of effort into it.

I'm inferring from your comment that one can't have basic knowledge of decent UI without being a designer. Surely a coder can have a good idea about how to lay out a decent interface without having the skills to actually do so? It's this missing skill that a theme gives them.

That's an ironically sweeping statement to make. For example, are all UX specialists designers... or does it just seem easy to designers ;-)

2. "Are all UX specialists designers?" depends entirely on how we define our terms. If we define UX as a design discipline (as is common, since it basically is), then yes, by definition. If we define it as something else, then no.

Making that attractive is a completely different skillset and one I certainly don't have. However, based on the amount of flowery yet wholly unusable crap I come across, it seems many designers don't really have a good handle on usability either. Getting someone great at both is a rare find indeed, but it's not as if there's this huge cognitive gap between the two.

And again, I contend there are people that do both extraordinarily well. And then there's a ton that think they can, on both sides of the fence.

The fact is that a coder CAN get a working product up without the design-equivalent of "coding". They can lay out a UI just as well as an average professional designer (I would know), but the execution will certainly fall short of an average professional designer (I would know).

With that said, I'd always prefer to hire an excellent designer before building a UI, but I rarely will because I like to build more than I like perfect UIs.

My experience says mostly no. They're different skillsets. Occasionally they overlap in an individual.