In reply to KTS915.

I think when they say "system font", they mean fonts that are native to the OS, not necessarily the users chosen font. I imagine they do this for a couple of reasons.

One being that if a user sets their system font to Georgia at 24px, that would probably screw up all sorts of spacing issues within WordPress, thus potentially making WordPress look bad, even though it was the users fault for choosing such a weird configuration.

Two, because it helps keep the experience more consistent across devices. It's not the exact same (like it is now with Open Sans) but it's close enough that people shouldn't get confused when switching devices.

Also, you can still load your plugin and override those CSS rules. It might not need any tweaking at all! Check it out against the development version. :)