.crm-form-select {
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
  background-size: 10px auto;          /* on réduit le chevron */
  background-position: right 0.6em center;
  background-repeat: no-repeat;
  padding-right: 1.8em;                 /* place pour le chevron */
}
#additional_participants {
  min-width: 4.5em;
}