The ~TiddlyWiki core adds several `attributes` to the rendered content. With those attributes it's possible to apply custom styles to the tiddler content.
For example this tiddler is named: "{{!!title}}" so the attribute looks like this:
To create a green border for every tiddler that starts with `$:/` aka system tiddlers, you'd need to use CSS like so: (not applied here but you can experiment with it! )