One of the new features introduced in Windows 7 is the use of themes to customize the system's appearance in a very easy way. Themes enable you to personalize wallpaper, screensaver, window color and system sounds in just one click. Then, if you want, you can change small details inside the selected theme and create a new custom version of it.
Themes can be selected from the Personalization window, which you can open by right clicking anywhere on the desktop and selecting Personalize. Switching to a new theme is as easy as clicking the theme's thumbnail. The theme will be immediately applied to your system, so you get the full preview before formally applying it. You can then click on any of the customizable elements at the bottom of the window (desktop background, window color, sounds and screensaver) and change them to suit your taste.
The best thing about themes in Windows 7 is that you're not limited to the ones that come shipped with the operating system. You can download more from Microsoft by clicking the 'Get more themes online' link in the Personalization window. Simply click the download button under the theme you want to use and download the .themepack file. Once it's on your hard drive, double click it and it'll be automatically applied – and added to the Personalization window under 'My themes'.
Of course, Microsoft is not the only source of themes for Windows 7. A quick search on Google returns dozens of sites to download themes from and try new styles on your system, like Paul's theme packs, the iThinkDiff collection or the themes in the IntoWindows blog. They're as easy to install as the ones you download from Microsoft.