Regarding the product, themes and customizations will most likely be the best option to maximize the usability for each individual user. This way, users could choose high contrast themes if they are not happy with the default.
As for the general design question you are proposing, my opinion is that there is no one-size-fits-all approach that pleases and maximizes the usability for everyone. For example, making all text larger might increase readability for people with below average eyesight, but will punish users with normal eyesight by decreasing the information density. I think that adaptive design, which software can uniquely offer to some degree, is the only hypothetical solution. However, this is obviously limited by the context-awareness of systems in that it doesn't help that a system could automatically adjust it's font-size if it does not know the state of the users' eyesight.