Color Styles
Reason
If the designer's does not specify a color style with a background-color style, and vice-versa (a background-color style with a color style), then the visitor's browser might fill in the blank with some undesirable effects.
The result might be ugly. Worse, if the visitor's color preference and the designer's background-color style are both dark or both light, then the page content might be invisible.
By specifying a background-color style with each color style, and vice-versa (a color style for each background-color style), the designer avoids those problems.