.btn-square-so-pop {
  position: relative;
  display: inline-block;
  padding: 0.5em 0.5em;
  text-decoration: none;
  color: #FFF;
  background: #999900;/*色*/
  border-radius: 0px;/*角の丸み*/
  box-shadow: inset 0 2px 0 rgba(255,255,255,0.2), inset 0 -2px 0 rgba(0, 0, 0, 0.05);
  font-weight: bold;
}

.btn-square-so-pop:active {
  /*押したとき*/
  box-shadow: 0 0 2px rgba(0, 0, 0, 0.30);
}