The old Print Preview actually allowed you to inspect the page with the Developer Tools. Which was great for debugging print-specific CSS. I don't think that's possible with the new Print Preview, but the new one is better in every other way.
The Browser Toolbox is the normal DOM inspector, just extended to the whole user interface. If you open the print preview, then open the Browser Toolbox, you can inspect the preview and edit styles just like any other document.
Also, @media print {} works fine, I just tested it. But I'm not sure how that relates to the original question.