I’m sorry, but there is no easy way of setting a custom breakpoints in the theme. Please understand that when building a theme that is used by many sites I need to make compromises, and one of them is using a device specific breakpoints. When building a custom theme for a specific client, it is preferred to use content breakpoints, I understand that, but it’s not possible in globally available themes due to I have no control/prediction of the content size.
But it should be perfectly possible to override the breakpoints with custom CSS :)
I strongly do not recommend editing theme core files to keep the theme updates available.