Setting Firefox to Dark Theme is only marginally useful when the actual web pages are unaffected by local settings.

This add-on makes my browsing experience far more comfortable over extended periods of time.

As noted in the description; there are technical challenges that lead to one or two quirks in display on some pages. These are sufficiently negligible and/or rare that it is no issue to work around them compared with the huge improvement in long term viewing comfort.

I wish I had found this add-on earlier.