> the CSS files in Kareha's mode_image have a lot of old unused elements still define

Once again, the CSS for mode_image is the same as both Wakaba and Futallaby uses. It's not supposed to be perfectly tailored to Kareha, and I won't be doing any tweaking for that.

> and THREADS_LISTED still exists in config.pl,

All options exist in config.pl, even those that don't apply to the current style.

> Error template div/body class in mode_image still missing -- on a side note, what font-size and/or font-weight would be appropriate to match h1 text styling in CSS?

I decided that since neither Wakaba nor Futallaby have it, I'm not going to add it, for the same reason as above.

> This might require its own sub function that basically removes all such limitations (text/file posts, permasaged/closed threads, choice of whether to use ID, etc).

Because it's a lot of work for something that will hardly ever be used, and really isn't necessary for anything.