This is very hard to fix via CSS.
This can very easily be fixed by modifying the plugins img URL.
Youtube supports multiple types of image for each video, if the dev could add support for hot swapping what image is used that would be great.
So to fix this…
find all references to “$img”
CHANGE: i2.ytimg.com
TO: img.youtube.com/
AND
CHANGE: 0.jpg
TO: mqdefault.jpg (this image has no border :])
Here is a list of all the images youtube uses.