In 2016, when Trump won, the way to protest ( if you couldn't in person ) was to black out your social media backgrounds to promote " #notmypresident" in reference to Trump.

Now, the only thing I can find, dated recently:

"it’s part of an update to make the Twitter website respond to the light and dark modes on your OS"

https://www.theverge.com/2021/2/3/22265306/twitter-dark-mode-dim-lights-out-black-darker-light-settings

Suspect at best though. Very curious though as to why Bongino is doing this as well.