Overleaf/services/web/frontend/stylesheets/bootstrap-5/pages/editor/write-and-cite.scss

.btn.reference-manager-search-group-dropdown-toggle {
  font-weight: normal;
  text-align: left;
  display: flex;
  justify-content: space-between;
  border-radius: var(--border-radius-base);
  cursor: default !important;
  background-color: inherit;

  &::after {
    display: none;
  }

  &:hover,
  &:active {
    background-color: inherit !important;
  }
}

.reference-manager-search-group-dropdown-icon {
  .material-symbols {
    vertical-align: middle;
  }
}