[Jetpack Plugin] Editing the copy of the comment field for posts

We’re using Jetpack for our comments and we want to change some of the copy of the comment fields. For example, right now the comment field has greyed text which says ‘Enter your comment here…’ which we’d like to change. Likewise we’d want to change the comment post button copy from `Post Comment’ to something else. Is there any way we can change this copy via CSS or some other way?