samples/showcase/src/com/google/gwt/sample/showcase/client/content/cell/CwCustomDataGrid.gss

.childCell {
  padding-left: 30px;
  border: 2px solid #eee !important;
  background: #eee !important;
}

.groupHeaderCell {
  padding: 1px 15px;
  color: #4b4a4a;
  text-shadow: #ddf 1px 1px 0;
  font-size: 130%;
  background: #eee;
}