gtk/demos/widget-factory/widget-factory.css

.small-button {
  padding: 0;
  outline-width: 0;
}

.border-inset {
  border-style: inset;
}

.border-outset {
  border-style: outset;
}

.border-groove {
  border-style: groove;
}

.border-ridge {
  border-style: ridge;
}

/* These effects use 2 lines, so show them */
.border-groove,
.border-ridge {
  border-width: 2px;
}