blackout screen outside slide to keep correct ratio

[hanickadot]

I'm trying to hack it myself for my presentation, when I present on screen with different ratio (typically a macbook with its 16:10 ratio with presentation intended for 16:9 projector) I would like to be able to blackout screen top/down or left/right and keep ratio same in fullscreen.

Can someone point me what to change? Or if such option already exists. Thank you!

[LangDaniel]

If you have a background image you can do that by setting data-background-size="contain":
https://salferrarello.com/full-screen-image-in-reveal-js/