Add this block to the end of backoffice/assets/backoffice.css:

.bo-nav-group-title {
  margin-top: 18px;
  margin-bottom: 8px;
  font-size: 12px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: rgba(255,255,255,0.65);
  font-weight: 700;
}