how to change font settings? size, colour etc

You can add your CSS to the bottom of global.styles.css, and just be sure to keep a copy of your styles somewhere so that if you upgrade the theme later you can paste them back in. Also, there are a number of ways to change font sizes and styles through the theme configuration settings. I'd suggest referring to the AT theme documentation for details, as it is part of the base theme: http://adaptivethemes.com/documentation/extensions

Wouldn't it make sense to use local.css instead of adding to global.styles.css? Is there a reason not to use local.css now?

You can certainly create your own local.css. The Fusion base theme included some extra functionality to check for the existence of it. That is gone now that this uses AT. It's pretty easy for somebody to manually add a stylesheet so it didn't seem like that much of a problem. You can do it in the theme's .info file. Or, as I suggest, just add to the bottom of an existing stylesheet. And when you update then you need to copy that back in, or re-add the line to your .info file depending on your method of choice.