Dark theme is a hard task as not all (Non-App Store) Apps have the same Dark tone. So I've ended up with a wall of various shades of black. Still, it looks pretty good, and is easier on my eyes.
TextEdit is completely adapted to dark mode though, as far as I can see – it has dark window frames, buttons and so on, just like you'd expect. The actual content area isn't going to be affected by dark mode (i.e. it won't turn from black-on-white to white-on-black, if that's what you are referring to) since it would break consistency and most likely also Apple's interface guidelines. TextEdit is a What-You-See-Is-What-You-Get text editor, the content on-screen isn't supposed to look any different in terms of colors than what it looks like when printed out (or when exporting it to a pdf, etc.). I haven't checked out help dialogue windows but it might be a similar story there.