Scroll Viewport gives you a different view of your Confluence space content. In Viewport especially in the default theme, most of Confluence’s styling in preserved. It is also possible to override the style of simple macros like tip, note, etc. but overall the content itself will be styled as in Confluence. Viewport themes focus more on the frame that you put the content in.
Scroll HTML Exporter
Scroll HTML Exporter exports the content of your Confluence pages into a static HTML file which you can upload on webserver. In HTML exporter individual elements like paragraphs, lists, tables can be adapted in the template and the template is the only source for styling. Most styles as set in Confluence are removed during the export process. Therefore, if you want to start from very basic HTML and want to do all the styling yourself, the exporter would fit your customization needs better.
Viewport works differently as it is a live view served by Confluence whereas the HTML exporter creates standalone static files that can be moved anywhere. The themes for the Apps are built differently and therefore aren’t compatible with each other.