$mainColor: green;

@mixin a_mixin {
  color: $mainColor;
  div {
    float: left;
  }
}

@mixin child {
  @include a_mixin;
  background-color: $mainColor;
  a {
    text-decoration: none;
  }
}

#main {
  margin: 0 5.625em;
  h5, h6 {
    font-size: 1em;
    line-height: 1.25em;
  }
  .post {
    text-align: left;
    ul {
      list-style-position: inside;
      list-style-type: circle;
      margin: 0.625em;
    }
  }
  .post-dash {
    @include child;
    color: red;
  }
}