Here's a working way to get dark mode. Opera Gx added a new feature where you can force sites to dark mode by right-clicking and select “Force dark page” on the site.
Because the sidebar uses the official Facebook Messenger site, go to messenger.com and force dark page it. Then your sidebar will also apply dark mode.

I hope this works for you guys too!

Here's a working way to get dark mode. Opera Gx added a new feature where you can force sites to dark mode by right-clicking and select “Force dark page” on the site.
Because the sidebar uses the official Facebook Messenger site, go to messenger.com and force dark page it. Then your sidebar will also apply dark mode.

I hope this works for you guys too!