It was relatively good until recently, when it was automatically updated, and after that I could not see the bookmarks in the bookmarks sidebar, nor the menu bar. All was just black, except for the icons for the bookmarks in the sidebar. It became useless
Eventually I figured it out.
So here is how it should be explained:

Open the settings of the Dark Reader > Click the "More" tab > Click "All Settings" > Click/toggle "Change Browser Theme" so that it says "Default browser theme is active".

If you want some custom browser theme, install it or enable it AFTER all this procedure through Firefox Themes settings. You actually may need to uninstall and reinstall the Dark Reader extension and revert to the default Firefox browser theme in the Firefox settings before all this.