I find the “Developer” easiest to read, but it reminds me of dull office furniture even more than Default does. Sorry to sound cranky, but I don’t like any of them much.
Oddly and annoyingly, when I switch between themes I sometimes, but not always, get thrown back to an earlier page rather than the one I was looking at when I changed the theme.
The “Forum Home” page shows some extra information (tables of “most recent” and “most popular” threads)in the Developer theme that is not shown under the other themes. These might be useful, but I’m not sure if they’re implemented correctly; when I posted to this thread it did not move into the “most recent” table.