I want to add a new page to my responsive website. Invariably there are additional CSS entries that are generated on the page, and I may have modified my previous CSS file slightly on the website. If I replace the generated CSS file, my existing website is missing some entries and/or modifications. Is there an easy way to prevent this from happening?

It has been recommended 'not' to alter the CSS files that RSD generates. The best practice is to create a separate custom CSS file and add it to your Resources. That way, when making new pages, or alter existing pages and then Export from RSD, you won't lose anything. Your custom CSS file will stay in tact and the CSS files that RSD generates will have the changes they need to make your new pages function correctly.