WordPress 4.6 has switched to system fonts.
A plugin for WordPress that replaces Open Sans with your device's standard user interface typeface.
There are other plugins that remove Open Sans, but they rely on the generic
sans-serif fallback. This one adds a new stylesheet that specifies the system UI font for most major operating systems.
Credit to this WordPress forums thread for the code that removes Open Sans. https://wordpress.org/support/topic/remove-open-sans-and-add-custom-fonts
Disclaimer: I never claimed to know what I was doing; if your site is hacked by pirates after installing this plugin, it's not my fault.