Ron and I had a quick idea and a mini challenge for the day. The result is this quick and nifty plugin that left me wondering why nobody had done it yet.
This plugin add the css styling needed for the wp-signup page so the form looks a bit better. It should work on any theme, barring any funky styling. I tested it on four themes and only one looked wonky (because they used display:inline on #content). But even that looked better than nothing.
This way, you can fiddle around with themes on the main blog without worrying about future theme upgrades or manually adding in css to the theme.
I also included the default css that is hardcoded in wp-signup should you wish to edit the plugin and override it.
Added bonus: this will only add itself to the signup page. It won’t be added to any other page on the site. You can put it in the mu-plugins folder, or the plugins folder. It’ll still work.