phpWS supports themes. Most CMSes do. The concept is pretty simple: Seperate, how the page looks and -- to a certain extend -- behaves from the rest of the content. And make that look and feel customizeable.

Compare a theme to, say, a WinAMP skin. Or your GUIs Theme, be it KDE or Windows XP. OS X, on the other hand, looks pretty much like Aqua all the time. The theme there seems to be fixed, which isn't as bad as it sounds.

You can and actually should customize your site theme, so it can be recognized and differentiated from other sites. On the other hand, you usually probably won't change your modules, unless you are developing yourself. Modules are autark, they handle stuff by themselves. Still, they allow you to make theme specific templates. More on that later.