The site could be build by hand or any editor then converted into a theme. It really does not matter which editor was originally used to create that theme. I've look at the HTML and you should not have any problem making edits in DW if by edits you meant text, images and such.
Keep in mind that elements defined using an absolute path such as CSS styles will not be editable unless you download a copy from the server.
Despite all that it's not an impasse to make changes as long as you have all the files required locally so you can make tests before committing those changes.