Bookmarks are not part of the DOM, they are only in the client's browser, therefore there is no CSS-selector to engage with in that way. There is also no way for javascript to connect properly to bookmarks in a user's browser (short of a browser extension), so you can't even really create a script that would be able to parse all the links in the page, compare it to the saved bookmarks, and add CSS to them, but then even if it were possible, that would be a heck of a script to write just to make a link look colourful.