Where is the right place to ask about installing the extension on new wikis?
Topic on Extension talk:DarkMode
I don't think it will be deployed on WMF wikis. Vector 2022 will soon have a dark mode option. For other skins, you'll have to rely on the gadget counterpart if it's available on said wiki.
Aha, thanks! How to get the gadget installed here / on Meta?
Just ask wiki admins to create those six MediaWiki: namespace pages?
I couldn't find any open Phab ticket about this. I think that the specifics of [soon] and implementation details of the option will leave dm-enthusiasts using the current gadget for a while.
On Meta, we should probably propose it first at m:Babel. Barring objections I can install it there. mediawiki.org doesn't have much of a "community" for things like this, so I might be bold and just install it. It's an opt-in gadget, after all.