For vertical screen games like this, I think it might look better if you move the logo to the bottom left and make the video a bit bigger to fill the void.
Anyway, those are cool theme videos, keep up the good work!
Cool theme, 500MB for a minute of video is a bit too much though, i'd suggest you to run it through Handbrake to make it smaller (encode it with a modern codec like x265 or av1, x264 is ancient at this point), also you gotta do something about the 30fps, it would be much cooler if you could make them 60.
That looks like that by design, it's meant for 4:3 snap videos, not for 16:9 theme videos. You might want to switch to a different BigBox theme, For a setup with lots of theme videos, I'd recommend the Unified themes (any of them would work).
The only games i have problems with are the ones on custom xci format that have multiple DLCs and updates all in one file (I get an infinite loading screen). other than that it works fine. NSP files and launching XCis from Yuzu itself works fine though.