$isActive: false;

@if $isActive {
  $color: 'red';
}

@if $isActive == false {
  $foo: 'bar';
}

@if ($isActive == false) {
  html, body {
    font-size: 14px;
  }
}

button {
  color: $color;

  @if $isActive {
    font-size: 13px;
  }
}