It is possible that your application will have  some kind of UI that requires some collection of icons, colors, etc.

Usually this may be in the form of toolbar button icons, but you may want to do more to control the look of your application.

As we saw in the previous example, you can attach CSS style sheets to XUL documents, and in these style sheets you can easily set toolbar button icons for example by using the list-style-image property.

You can also do more elaborate things – it is an XML document and thus is completely stylable using the W3C CSS standard as implemented by Gecko.