I would have thought that someone with a visual impairment would be best off setting up their browser to have nice large fonts, rather than relying on individual websites. Though I admit that since I have no problem in this area I'm not in the best position to comment.

If you really do want to alter your website to have larger headings then at the moment this isn't possible without building your own custom theme.

However note that in the next week or so I hope to put a change onto Voice that allows you to specify extra CSS for any existing theme. Once this is live you could use this feature to add a rule to make all your headings larger.

Thanks

Joe - Voice Admin