cssValue method 
    
    
    
  Implementation
  @override
String cssValue() {
  var str = '';
  if (color != null) str += 'color: $color ;';
  if (backgroundColor != null) str += 'background-color: $backgroundColor ;';
  if (fontStyle != null) {
    str += 'font-style: ${EnumToString.convertToString(fontStyle)} ;';
  }
  if (fontWeight != null) {
    str += 'font-weight: ${EnumToString.convertToString(fontWeight)} ;';
  }
  if (borderColor != null) str += 'border-color: $borderColor ;';
  if (borderRadius != null) str += 'border-radius: $borderRadius;';
  if (padding != null) str += 'padding: $padding;';
  return str;
}