Yes good question, DM or a Note is almost the same thing. You leave something for someone somewhere. I prefer to keep it separate, but that's personal.
Making your own notes I get, that's easy, but why not have it in its own Notes section just like iOS has? That way you keep better track of what you're doing where. Now it gets mixed up. Still, XF2.3 is becoming beautiful this way.
Then you are able to easily find the solution mentioned by @Mr Lucky
https://xenforo.com/community/threads/dark-mode-light-mode-switcher-button.198154/post-1639741