How edit/style MarketPress pages?

I apologize if this is dealt with somewhere & I just haven't found it...

I'd like to know if it's possible to edit/style the various MarketPress pages (checkout etc.) without losing my changes when there's a plugin update. I have MarketPress installed and it's working great, but those pages look rather unformatted, no matter what built-in theme I use. I'm talking about the pages from checkout thru to order completion, the order tracking page and even the shopping cart widget.

Maybe I'm just not seeing the woods for the trees?

Something else I'd like to know is whether I could change all instances of "Tax" or "Taxes" to "VAT"? I suppose I might have to do a translation to achieve that?