Just thought I'd pop in and mention that forum themes *can* be changed client-sided- if you can code CSS, you can do something called a "user stylesheet," and if you're impatient/don't know how to use CSS, there are browser extensions that can take care of that for you (and manage uploaded override styles, too. Using one right now, in fact).
Can't really do anything about layout, though. CSS just handles the graphics style (colors, button images, etc). The content is HTML generated by PHP scripts from SQL databases, so while it's technically possible to make a re-interpreter to change the layout, it's just not worth it.