I’d love to be able to have the option to use bold, italic, and underlining on index cards.
I’ve just had a try within the Inspector on different tabs and was unable to change format–as you report. It looks like a feature that’s unavailable at present on the Windows version. Your only option would be to use upper case if you want to emphasise a word or phrase, perhaps.
For what it’s worth, synopsis formatting is unavailable on Mac as well. I’m not L&L staff and can’t speak for them, but as changing this would require changing the project format as well as updates to all the applications (Win, Mac and iOS), this is unlikely to happen any time soon, if at all.
The Synopsis field is plain text, and therefore not format-able.
One “solution” might be to use Markdown formatting (italic, bold, etc.). Not perfect, but perhaps better than nothing.