These STYLE statements create four style elements. Each style element is composed of attributes that define the foreground and background colors as well as the font. Notice that the style attributes are defined in terms of the user-defined attributes that were created earlier in the style definition. For example, the foreground color in cellcontents is set to colors("light"). Looking at the definition of colors, you can see that this is white. However, by setting the colors up in a style element with user-defined attributes, you can change the color of everything that uses a particular color by changing a single value in the style element colors. 
      style cellcontents /
         background=colors("dark")
         foreground=colors("light")
         font=fonts("cellfont");
      style header /
         background=colors("medium")
         foreground=colors("dark")
         font=fonts("headingfont");
      style systemtitle /
         background=colors("light")
         foreground=colors("bright")
         font=fonts("titlefont");
      style footer from systemtitle /
         font_size=3;
      style table /  
         cellspacing=5 
         borderwidth=10 
         bordercolorlight=colors("medium")
         bordercolordark=colors("dark");