All encodings are already supported and there isn't any* issues with encoding. InputStream is used and passed to XML parser because then it will automatically detect encoding. If we would use InputSource then when reading XML from file we would have to know what encoding is used, to build InputSource, but now we just leave it to XML parser. Now here we actually could use InputSource as we already have String, but then we would need 2 `parseTheme` implementations so I think easier is just convert here `String` to `byte[]` and let XML parser handle it.

*it's not backwards compatible because in store old themes have "<?xml..." with utf-8 encoding specified but we're passing it as utf-16 to parser and so it bails. Now that I think there isn't any reason to use UTF-16 and it should be UTF-8 because that's also default encoding for XML without encoding declaration. It will also be backward compatible.