Recently, SE switched to system fonts which, for many users, has caused a design problem that affects the readability and accessibility.
The new fonts seem to complement only certain browsers and operating systems, and are a setback on a lot of others, especially on Windows machines. As has been said before, the SE users all need different settings to be comfortable. I'm therefore proposing that this should be made an option in our preferences, or that the design team consider different fonts. It shouldn't be difficult, as this change hasn't affected sites with custom fonts; so please consider this.