Linux is the "generic" name for operating systems built on the Linux system kernel. Linux comes in a variety of "distributions" that provide different interfaces and program, while providing the same underlying system services. The most popular versions are Ubuntu, Fedora, OpenSUSE, PCLinuxOS, and many others. Here's a site that provides all the distro information you need:
http://www.distrowatch.com
Linux is an operating system (like Windows). It won't screw up your PC or prevent programs from working, because Windows programs don't run on Linux distros. There is a native-Linux application for just about all Windows apps. Some apps, like OpenOffice, Firefox and others, have versions for Windows and Linux.
You can download installation images of Linux distros from the liks at the above sites. You then burn the image to a CD or DVD (depending on the distro) and many will boot up and run as a "Live CD" directly from the CD or DVD, so you can see hos things work. This installs nothing on your system.
If you decide to try it out fully, you simple click an installation icon or select something from a menu and the distro can be installed on your hard disk. You will be provided the opportunity to install the distro on a separate disk partition. The installer wil also usually install a boot manager/loader, which will allow you to select Linux or Windows when you boot the system.
The themes are usually designed for the specific distribution or desktop environment. The theme you're interested in requires Gnome, so your distro would have to have that desktop environment loaded for you to use it. Most distros use either Gnome or KDE. Most can use both, and you can select the one you want when you log into the desktop.
There's a ton of Linux information out on the net.