Themedreamer is a great php-wysiwyg-tool. (it doesn't work with Artisteer-template though). You will find it on mac-bb.org and macserialjunkies.com. (you have to register, just like here)
Otherwise: just view a installed WP theme which comes close to what you want in the browser, click on view source, copy all code, make a new html-page in DW, delete all code, paste code from wp-theme, correct link stylesheet and you can customize all css in wysiwyg-mode. Then -only- upload the customized css into wp. Done.