getPadding method
It'll provide CSS margin code.
Implementation
String getPadding() {
  if (all != null) {
    return "padding: ${getSizeWithUnit(all, sizeUnit)};";
  } else {
    return """padding: ${getSizeWithUnit(top, sizeUnit)} ${getSizeWithUnit(right, sizeUnit)} ${getSizeWithUnit(bottom, sizeUnit)} ${getSizeWithUnit(left, sizeUnit)};""";
  }
}