Adds a style class to @widget.
After calling this function, the widgets style will match for @css_class, according to CSS matching rules.
Use [method@Gtk.Widget.remove_css_class] to remove the style again.
The style class to add to @widget, without the leading '.' used for notation of style classes
See Implementation
Adds a style class to @widget.
After calling this function, the widgets style will match for @css_class, according to CSS matching rules.
Use [method@Gtk.Widget.remove_css_class] to remove the style again.