Ok I know 3.1 is responsive but my main style is quite 'busy' and Id like a much simpler style when on a mobile device.
So it switches to the simple style on mobiles but normal style on desktop. Is this possible like it was on 3.0?
It was not default in 3.0, you needed a modification for that. Mobile detection nowadays happen via css (window width), the only modification I remember did it afaik by browser agent detection with a quite long list of browser agents (but before you ask: I don't have a download source).
I know its a style for 3.0.x (so obviously will not work on 3.1.x and 3.2.x) but it has a mobile detect switch which you may be able to integrate into your idea. Whether it will work or not I have no idea.