CSS classes are declared using the
classes function in the
attributes list. It's a set of text values that is added to the
underlying GTK+ widget's style context as class names.
widget Button [ classes ["big-button"] , #label := "CLICK ME" ]
GTK+ supports a subset of CSS. To learn more, see the GTK+ CSS Overview.
To have any effect, a CSS provider needs to be set up for the GDK screen. See the CSS example in the repository for a demonstration.