gtk-style-context-add-class (context classname)

context -- a gtk-style-context object
classname -- a string with a class name to use in styling

Adds a style class to the context, so posterior calls to the gtk-style-context-property function or any of the gtk-render-* functions will make use of this new class for styling. In the CSS file format, a GtkEntry defining an "entry" class, would be matched by:

GtkEntry.entry { ... }  

While any widget defining an "entry" class would be matched by:

.entry { ... }