Writing on WordPress is in many ways similar to writing HTML 1.0. The amount of markup you use is about the same. Paragraph breaks, headers, lists, blockquotes are at your disposal. You have fewer types of lists than early HTML had, you don’t have the address tag, but you can add a caption to an image. Your options are slightly different, the amount of options is about the same. The rest—the actual layout—is done by the theme.
In the days of HTML 1.0, the layout was done in the browser. From the very beginning, browsers offered a detailed configuration for how they displayed web pages, most of them allowed you to set fonts and colors for every element separately, just like a modern style sheet. Default style sheets were often quite fanciful, look at the Cello style sheet as an example. AMSD Ariadna could apply 3D effects to headers. In modern browsers, these options are usually reduced, though some may allow you to use your own, separately written, style sheet. In general it’s not a good idea to mess with the default settings any more, since it may completely screw up some pages where the designers defined some but not all elements.
The big difference of course is that back then, the choice was the user’s, and now, it is the publisher’s, writer’s. This may flatter the narcissism of some writers, who want full control over how their products are presented, but it doesn’t make a lot of sense. Even if you think that the writer’s taste should supersede the reader’s, the problem of accessibility and device remains. Disabilities aside, some people have better eyesight than others, an iPad and a 24″ 16:9 display require completely different designs, and so does the amount of light in the room, one aspect that usually gets overlooked completely.
The way WordPress is built, a user choice of themes wouldn’t work very well, since applying a new theme to an existing blog usually does require some finetuning, and not just due to the different way different themes treat featured images. It would probably work better on Tumblr, where the post types are stricter. It would be interesting to see it implemented on any such platform.
In the meantime, we can just muse how some power went from the user to the publisher and the Web has become, just a little bit, more like TV.