The web today is designed using CSS. HTML itself just gets the content across. The CSS is what styles it into either something you love or some ugly website. That being said, you will need to have some knowledge of CSS if you want to create a high quality site. WYSIWYG editors don’t really manage CSS. Anyway, yes. Dreamweaver is a good option as a WYSIWYG editor for creating the theme. You can just copy the code out of the code editor in Tumblr and paste it into Dreamweaver. Here it will tell you how to make custom themes. It basically talks about the tags you need to add to the HTML. Dreamweaver won’t know how to interoperate the tags because they are not standard HTML elements, so don’t get confused.