Is there a way to change the default font (Helvetica 14px)?

You can define default styles for each element type per project using the 'type selector'. So for a specific project you could set all paragraphs to use Roboto 17px for example and every time you add a new paragraph these styles will be applied by default.

So if I add a Paragraph, then select Apply To 'Type', my subsequent changes in Design will modify the default settings for Paragraph?

For my static site, I included default settings for body and table text so I would not have to apply a class every time I added new strings. Using RSD, I will have plenty of text included from other sources and I'd prefer to use the CSS generated by RSD first, then any of my custom CSS, second.

I hope I'm making sense. It sounds like I should go through most of the elements, then apply my preferences to the corresponding Type in Design.

Correct. A good workflow is to start applying common styles to the 'type'. That way every new element takes these styles by default. You can then apply alternative styles — style variations — to classes and very specific tweaks to IDs.

Note, IDs selectors are more specific than Classes selectors and Classes more specific than Type selectors. The more specific selectors will always prevail over less specific selectors so if you already have styles applied to classes, setting styles to the 'Type' will not change existing elements. You can however reset styles (for type, class or ID) with the refresh button next to the selector drop down.