Lead Developer
Hi Naomi,
At this time, you would have to edit the core mobile.css file and change the break point manually – not a big deal, but updating the theme would change it back.
I do have plans to make it much easier to change the break point – it will hopefully be in GP 1.2.9.
As for the second question, you can use your own media queries in your custom stylesheet to create new breakpoints and change CSS for those sizes – more info on media queries here: https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Media_queries