However you use index cards, within Scrivener, you’ll have to mark up your index card text another way. While you’re probably not interested in MultiMarkdown, one thing you can borrow from it is the way that bold and italics are marked. Rather than relying on a font to italicize, you simply surround the word or phrase with underscores. To represent bold, put DOUBLE asterisks around the word or phrase to set it apart.
Technically, it’s the doubling of either _ or * that makes something bold, so that this is also bold, but I like to mix it up visually, and underlines are linked to italics in my mind, so the underscores look like underlining to me. You can ignore all of that though, if you’re not going to use MMD, and just use something like this to represent various formatting. {Curly braces} or [square brackets] might also work, or +plus signs+, or any of a large number of symbols that you can produce with your keyboard using the OPT key and SHIFT-OPT keys in combination with most letters & symbols.
There’s no right or wrong way to use the elements of Scrivener, but there are limitations to how each element can perform.