static/themes/metro/splitbutton.css

.s-btn:hover .m-btn-line,
.s-btn-active .m-btn-line,
.s-btn-plain-active .m-btn-line {
  display: inline-block;
}
.l-btn:hover .s-btn-downarrow,
.s-btn-active .s-btn-downarrow,
.s-btn-plain-active .s-btn-downarrow {
  border-style: solid;
  border-color: #b3b3b3;
  border-width: 0 0 0 1px;
}