A funny thing about TiddlyWiki is that it seems to be mostly undocumented.

I don’t like the default style where text is not justified on the right. It violates typographical rules.

Like many settings in the wiki, the styles are actually very simple to edit: they’re contained in the shadow tiddler StyleSheetLayout. Edit that tiddler and add “text-align: justify” to the .viewer class, and you’re done.