Well, HTML export uses CSS mainly to reduce the size of output - they are generated automatically.
The idea of passing css is that those automagically generated CSS styles could be shared across the group of files. Before the first file, Index should be empty. Later convert it to HTML using "AsCss".
Links are "translations" of RichText links to HTML links. You can leave it empty if you do not want to have links translated.
BTW, instead of SaveStream, I would use SaveFile here