#cateprincipal, #subcategoria { width:296px; text-align:center;}
select { width:240px; 
display: block;
  padding: 8px 6px;
  margin: 0 auto;
  position: relative;
  cursor: pointer;
  border-left: 10px solid #739cda;
  background: #fff;
  font-weight: normal;
  box-shadow: 1px 1px 2px rgba(0,0,0,0.3);
  transition: all 0.15s linear;
}