iframe issue Milk V.2

I have made a minor change to the base.css files (iframe) to add a small border around iframes and a little radius to round the corners.

But the really strange thing? Instead of a border around a YouTube video for example, it puts the border inside the video. I’m using PHPbb media embed for the video placement.

All other iframes (so far) seem to be perfect.
You can see it here:
(link removed)

The right hand side shows how it should look, the youtube video in the post shows the problem.

Sorry, these are the changes.

     border: 2px solid white;
     box-sizing: border-box;
     padding: 5px;
     border-radius: 10px;