If you'd like to add padding to a document exported as a horizontal slider with scaling, you could export the original document with Desktop Scaling applied and then embed it into another document using the in5 Web Content interactive widget. Here's how:
Sounds good.
If you want to add padding via CSS, you can add your own custom CSS using the Resources section of the Export HTML5 with in5 dialog.
I checked with Justin for some CSS. He said that you could try something like:
body { padding:40px; }