On GTK 3.20 onwards, themes are expected to use CSS names rather than widget class names, and apparently CSS names are not inherited. So, a derived widget altering its CSS name won't be styled as its parent classes. Thus, SnItem setting the CSS name "sn-item" avoids being styled as a "button". But on older GTK versions, the widget class being used makes this change ineffective. Fix that by introducing a custom derived button that doesn't draw the unwanted parts.