For your mixin to work, it should be called like this:

.panel-light {
  .sec;
  background-color: transparent;
}

So Your Corrected Code Should Be:

.sec {
  border: solid 1px black;
  border-radius: 10px;
  padding: 10px;
  margin-bottom: 10px;
}

.panel-light {
  .sec;
  background-color: transparent;
}