I also could not find a way of changing the defaults in Word Online.
Apparently "MS knows best" and sees no need to allow users to customize their default document in Word Online.
You could use a workaround. Create a new blank document, make the changes you want then save it (with no text in it).
Now when you want to create a new document in Word Online with the same attributes. Next time you want to start a new document, find your "template" [(ab)using the term very loosely], right click on it and select "Copy To". Pick a OneDrive location and you will have a new blank document with the attributes set.
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Normally I would suggest submitting a "Feedback" to MS, but in this case it is a waste of time. Granted, MS acts on relatively few feedback submissions, but they really do act on some. Unfortunately, in this case, Word/Office Online is considered a "loss leader" or "bait and switch" tool. The Online apps provide minimum functionality and MS is unlikely to add much to it. The free version is just there to hook new users on Office and provide read and simple editing tools for people working from mobile devices when they don't have access to the desktop version.