I use https://github.com/darkreader/darkreader to optionally apply dark themes to websites that don't support themes, or don't provide theme options. It's pretty good, but there are definitely some sites that it doesn't work well enough with. Worth having though, as I do prefer dark themes - I feel my eyestrain has been reduced since using it.

Perhaps this type of functionality (perhaps limited to toggling the prefers-color-scheme setting) is something browsers should consider baking into the default UI/Chrome.