Marketpress Styling

I've been trying unsuccessfully to modify some of the marketpress styling and make it a little more compatible with my existing styling. I have a few questions....

1) I am trying to override the styling for the images shown here ( to remove the shadow. I can't seem to override and if I update the actual file (icons.css is what im using at the moment) do I lose the changes in the next update?

2) The checkout page has some truely dreadful old icons on it (, same as before, can these icons be overriden? The paypal icon appears to be generated by paypal (<img src="" alt="PayPal">:wink: when I know for a fact there are much, much prettier icons that could be used. Where and how would I update these without losing the changes during an update?