We really appreciate you giving us this feedback.
I've removed the sidebar from the dark theme. It should be replaced by discourse' regular navigation tool.
I queued up for development resolving this issue
It appears the sidebar refers to specific colors, rather than the css styles that change with theme. If that's the case, it should be a quick fix. I'll reactivate it once it's actually visible to dark-theme users.
Dark theme also doesn't play well with our custom banners and top logo. So I added that to the development list as well.
Also TIL, 150 users have selected dark theme.