Hi, I can't figure out how to change the padding either, it looks to be built in and not user changeable thorugh the UI - although there may be a way to do this outside of infopath (editing the source files, but that can break your form is you are not careful).
To change the background color go to the "General" tab of the page properities and check the box for "Use a Background Color" and choose white. The Grey color is the InfoPath 2010 color by default, that is the "automatic" colorg. You maybe able to change this default automatic color somwhere but I don't know.