example.css

.filtros-h { position: relative; }
.filtros-h select { width: 100%; }
.filtros-h .extend {
  background: #ffffff;
  border-top: 0 none;
  border-left: 1px solid #aaaaaa;
  border-right: 1px solid #aaaaaa;
  border-bottom: 1px solid #ffffff;
  height: 11px;
  position: absolute;
  top: 34px;
  width: 218px;
  z-index: 9999;
  display: none;
}
.filtros-h ul.options {
  border: 1px solid #aaaaaa;
  list-style: none;
  width: 928px;
  max-height: 400px;
  overflow: auto;
  position: absolute;
  padding: 3px 5px 10px 5px;
  display: none;
  z-index: 1;
  left: 10px;
  top: 45px;
}
.filtros-h ul.options-open {
  background: #ffffff;
  z-index: 999;
  -moz-box-shadow: 0px 3px 3px #e9e9e9;
  -webkit-box-shadow: 0px 3px 3px #e9e9e9;
  box-shadow: 0px 3px 3px #e9e9e9;
}
.filtros-h ul.options li {
  float: left;
  width: 268px;
  padding: 6px 10px 6px 25px;
  border-bottom: 1px solid #efefef;
}
.filtros-h ul.options li a { color: #333333; }
.filtros-h ul.options li.active { background: url(img/icon-v.png) no-repeat 5px 7px; }
.filtros-h ul.options li a { display: block; }
.filtros-h p.select-label {
  border: 1px solid #aaaaaa;
  padding: 7px 25px 7px 10px;
  cursor: pointer;
  -moz-box-shadow: 0px 3px 3px #e9e9e9;
  -webkit-box-shadow: 0px 3px 3px #e9e9e9;
  box-shadow: 0px 3px 3px #e9e9e9;
  background: url(img/drop_down_arrow.png) no-repeat 95% 50%;
}