Just a suggestion, and I'm sorry I haven't the time to describe how, but it would be a great idea to copy the theme first, adjust for large fonts/hard of seeing, then send to the theme creators to publish them alongside the originals... perhaps it could even be a standard thing... then the standard user can easily choose a "large font" version of any theme.
It's not a lot of extra work for the theme designers really, and give them an additional market that might consist of people who don't care too much about the theme itself, but do want their phone to be easily readable..