The UI is fairly boring and has limited customization options. There is a dark theme available, however it only applies to the editor. The surrounding windows and borders remain light. You can see a collection of screenshots here.
NetBeans is a free, GPL-licensed IDE. It can can run on any computer with a Java virtual machine. If a computer has a Java virtual machine (JVM), Netbeans can run on it. Netbeans can therefore run on a variety of operating systems such as Windows, *nix, and Mac OS. EditComment References 1