If you practice a bit and learn some simple rules of composition, typesetting, and color, you can be a competent designer. Beyond that level you need native talent and drive but there is no excuse to be ignorant about related fields.
Sure they can, but are you saying there is no difference between the guy playing guitar at the local subway stop and Eddie Van Halen?
Design is a skill just like programming is a skill. To suggest that everyone can be equally talented in a particular element is to demean the trade and the people in it.
Maybe. But I would argue that most people's talent is the result of persistent hard work in that discipline. And that is a compliment.
Sometimes people use "I'm not a designer" as an excuse for why they aren't good at design, as if it's something you need to be born with. It could be that they've just never cared enough to try, but to make themselves feel better they dismiss it as an impossibility.
When you're looking far up the ladder it's tempting to call it a gift at birth--when you're only a notch below, it's easier to see it as the result of hard work. What drives those people is another question.
There's a huge difference between being able to tell if something looks good and being able to design something that looks good.
I think for most programmers that difference isn't a technical one. The basic idea behind CSS is pretty trivial compared to even a moderate programming task.
Personally, even if I knew CSS inside and out I doubt I would be a very good designer.