Not really a CSS question.
This is one of the rare cases of basic formatting where SugarCube's formatting markup doesn't have a direct analog. The easiest thing would be to simply use the HTML italics markup (<i>…</i>) around the story variable when printing it. For example:
The <i>$shipname</i> is your ship?!
If you're never going do anything with $shipname other than print it, then you could add the markup to the stored name. For example:
→ Instead of something like this
<<set $shipname to "Millennium Falcon">>
→ Do this instead
<<set $shipname to "<i>Millennium Falcon</i>">>
You'd print it as normal:
The $shipname is your ship?!
If you will, or may, be using it for other purposes, then a widget may be a good option. For example: (goes in a separate widget-tagged passage)
To use it, you'd simply invoke the widget in place of printing the story variable:
The <<shipname>> is your ship?!