Share 'Far too easy to accidentally change theme'

I've done a lot of advanced customisation on the commercial Ning site I'm creating. When in the 'appearance' page, a rogue mouse-click seems to have accidentally clicked a theme thumbnail before heading to the 'advanced' bit to customise the CSS. Nowhere did it warn me "hey, you're about to change themes and wipe all your customisation settings" - I hadn't realised I'd accidentally done so! So…