Black line around a thumbnail hover video

Hello all!
I’m trying to get rid of a black line around a thumbnail that has a video on hover.

The only thing i have found so far is this css code:

#content-holder .mejs-container { background: transparent !important; }

but it doesn’t work for videos that are used as a thumbnail hover preview.

I am no expert but i have a sense that it needs a small tweak in order to work. Any suggestions?

Thank you!