In standard CSS, you can't define a particular value (for example, a color or a font size) to "reuse" it in more than one selector you have to copy and paste it where you need it.

So, if you have to change it, you have to search for the value and manually replace it every time it occurs. As you can figure out, this is an error-prone process that can bring lot of problems and layout inconsistencies.

Moreover, if you need an interface that supports multiple sets of color and must be adjusted on the fly , you have to work with a lot of CSS files having the same declarations but different colors, and you would also have to maintain them for other updates.

The RichFaces skinnability feature is here to help us; it's not a CSS replacement, but integrates it by adding more capabilities.