I have a self hosted media server, and I want my family to use it more so I don’t have to do everything for them. I think the best way to do that is to have a wiki available on the local network where they can see a reference of how to use things. What is the best way to accomplish this?
Check out Bookstack. It's a brilliant piece of software that gets out of your way and has a modern look and feel that your family members will like and appreciate. The challenge will be to get them to use it instead of going straight to you. They may find themselves inconvenienced by having to go to a self-help resource. That's a them problem and not a you problem though.
Dark mode can be done by changing the template. You can install a new template, or there's an admin panel to change the colors, and you can get dark mode that way. But it's true that there's no dark mode for some and light mode for others option.
I remember using tiddly wiki a long time ago. Do you still prefer it over other solutions? I feel like my tiddly wiki would be too large of a file by now to be manageable, but I'm not really sure.
I'm currently running Dokuwiki as a Docker container, which has a built-in editor, good admin web-ui panel, easy ways to add multiple users, and also baked in access control rules so some can edit certain pages. It is also flat-file so no storing plain text in a database, so you can backup and migrate easily.
There are a lot of alternatives, but Dokuwiki is quite mature, and has that familiar Wiki look that your family might appreciate, rather than it looking like boring/corporate software documentation.
It really mostly depends on if other family members will need to edit the wiki.
If they'll be only reading dokuwiki might be best - even a single frontpage with all the info nedded.
If they will be editing then bookstack is way more friendly for non-technical people as is had a full WYSIWYG editor and you don't have to bother with markdown.