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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.

  [ 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.