Perhaps you’ve already got this figured out for Google Docs (this IS a rather old question), but I thought I’d help out just in case.

In Docs, create a new document. Go to Format > Document Settings. Change any settings in there that you want (Font, for example), then be sure to check the box at the bottom of the dialog that says “Make these the default styles for all new documents”. And that should do it!

I suspect AbiWord probably has a very similar process (some check box on a document settings screen), but I don’t use AbiWord, so I don’t actually know.