Yes, you can definitely update the css manually if you would like. In the css folder is a file called mystyles.css, if you add css to this file it should not get overwritten if you alter the css using any of the WebAssist tools.
If you are interested in updating the checkout to a UK supported one then you would need to make use of eCart. You would also make use of eCart if you wanted to adjust the shipping quote server behaviors that are applied to the confirm page.