[Lynne]

I believe the comma is added here:

.commalist li:after {
    content:",";
}

If you only want to change it for the showthread.php page, then you probably want to use an id when changing it like:

#tag_list_cell .commalist li:after {
    content:",";
}

and do that for other areas you want to change it.

For changing the separation as commas, I think you'd have to edit at least the function fetch_rendered_tag_list and rebuild_content_tags and possibly other functions also. Do a search in your files for "taglist" and see what comes up.