Hi Rodica,
Just updated my version of Neve (for some reason, it was only showing an update being available on the WP Themes page, not the Updates page). That seems to have mostly addressed the issue, but not quite fully — instead of massive letterboxing, the black bars above and below the video are now small, but still present — e.g.:
View post on imgur.com
This happens for any YouTube embed anywhere on my site (any YT video, embedded on any page, post, etc. on the site).
Thanks