Do mean just any corner, or do you mean the corner of rectangles? For rectangles (or squares), the Rectangle tool has the ability to round corners. It doesn't sound like it works the same as Illustrator though.
When you're using the Pen/Bezier tool, you can make round corners either as you draw them, or coming back later to round them using the node handles. Cusp/Corner nodes make sharp corners, and Smooth nodes make round corners. To set corner nodes, just click once. to set a smooth node, click-drag, and you can see the handles coming right out of the node.
Personally, I've never been able to control the smooth nodes enough to just drag them out as I draw a path. I'm not much of an artist. But some people are quite good at drawing round corners by setting smooth nodes, and adjusting them on the fly. It takes some practice, I'm sure.
On Linux systems, I have the impression that it's fairly easy to change the theme. But I don't think there's any way to change the theme with Windows. Well, not in terms of color. But you can use any icon set, if you find one. I thought I saw a set on the Inkscape website recently. Or you could always make your own, if you had enough time and motivation. I've made a feature request for a system of skins or themes which are easily installed and exchangeable. But I expect developers are more interested in adding vector graphics features, than themes.
Most Illustrator converts that I've heard of, seem to find it frustrating to use Inkscape. I don't understand why they think Inkscape should work the same way as Illustrator, since Inkscape does not strive to be, or promote itself as an Illustrator replacement. Inkscape does not have nearly as much fancy functionality as Illustrator, but it's still an excellent tool, and as far as I understand, it has some features which Illustrator doesn't.
There's a small section in the manual which covers the basic differences:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Illustrator.html
