The solutions provided in these related questions
do not work with the most recent update of MacOS. Specifically, the line
defaults write <Bundle-Identifier> NSRequiresAquaSystemAppearance -bool yes
does not have any effect in Safari. Neither does the app NightOwl, though it does work for apps other than Safari.
Does anyone have any suggestions on why this no longer works?
I use Gray, which is available on GitHub. It will do what you want to do.