full screen video without black bars

Isn’t it already applying object-fit: cover? The black bars are part of the YouTube video itself, so it’ll be a YouTube solution: https://support.google.com/youtube/answer/118277?hl=en-GB

How do I contact the developers of the twentyseventeen theme? I’ve searched for an answer to this problem and there are a lot of people having this problem, but no one can come up with a solution…

There should be some sort of override or something to make this work..

If you want to raise it as a bug to the core developers then create a ticket here and select “Bundled Themes” as the “Component”: https://core.trac.www.ads-software.com/newticket

For future readers, please do not create tickets in the above URL unless you have discussed your issue with a volunteer and concluded it was a bug with the core theme.

Let us know the URL of the ticket you created.