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input.scss

// ----
// Sass (v3.4.13)
// Compass (v1.0.3)
// ----

@function solidify($alpha-color, $background-color: white) {
  $percent: alpha($alpha-color) * 100%;
  $opaque: opacify($alpha-color, 1);
  $solid-color: mix($opaque, $background-color, $percent);
  @return $solid-color;
}

$color: #12B398;
$light-color: solidify(rgba($color,.2));
$inverse: white;

body {
  margin: 0;
  padding: 50px;
}

.follow {
  & {
    position: relative;
  }
  span {
    border-radius: 50%;
    font-family: sans-serif;
    text-align: center;
    display: inline-block;
    width: 40px;
    height: 40px;
    line-height: 40px;
    background: $inverse;
    color: $color;
    border: 1px solid $light-color;
    background-clip: padding-box;
    margin: 5px;
    user-select: none;
    cursor: pointer;
    transition: all .25s;
  }
  span:active {
    background: $light-color;
    border-width: 6px;
    margin: 0;
    color: $inverse;
  }
  input {
    position: absolute;
    visibility: hidden;
  }

  input:checked {
    + span {
      background: $color;
      color: $inverse;
      border-color: $color;
    }
  }
}

output.css

body {
  margin: 0;
  padding: 50px;
}

.follow {
  position: relative;
}
.follow span {
  border-radius: 50%;
  font-family: sans-serif;
  text-align: center;
  display: inline-block;
  width: 40px;
  height: 40px;
  line-height: 40px;
  background: white;
  color: #12B398;
  border: 1px solid #cfefea;
  background-clip: padding-box;
  margin: 5px;
  user-select: none;
  cursor: pointer;
  transition: all .25s;
}
.follow span:active {
  background: #cfefea;
  border-width: 6px;
  margin: 0;
  color: white;
}
.follow input {
  position: absolute;
  visibility: hidden;
}
.follow input:checked + span {
  background: #12B398;
  color: white;
  border-color: #12B398;
}

rendered.html

<label class='follow'>
  <input type='checkbox'>
  <span>+0</span>
</label>
<label class='follow'>
  <input type='checkbox'>
  <span>+0</span>
</label>

toggle-button-test.haml

%label.follow
  %input{type: "checkbox"}
  %span +0
  
%label.follow
  %input{type: "checkbox"}
  %span +0