Theming GUIs
What to Theme?
There are generally two things that people like to change the look of, what applications look like and what windows manager looks like.
Scroll bars, buttons, icons, and toolbars can be are all examples of application themes. Most Linux applications use one of two GUI tool-kits, GTK or Qt. GNOME and Xfce applications use GTK, and KDE applications use Qt.
Windows borders and backgrounds are examples of things that are themed with the window manager. To theme a window manager, you have to know which one you're using. GNOME uses Metacity, KDE uses Kwin, and Xfce uses xfwm4. If you are using a window manager, then it is the window manager.
How
Application Themes
Window Manager Themes
Most window managers will tell you how to change their theme in the documentation. Visit your window manager's website for more information.