Set default values in .CSS instead of .dart

e.g. `<b-loading>`:
Remove

``` dart
  String _defaultColor = "#505050";

  void attached() {
    // Setting the color to the default color in case no color
    // has been provided through the color attribute.
    if (color == null) {
      updateColor(_defaultColor);
    }
  }
```

and set default color in .CSS
