Why not use the dark rendering mode of your browser? Its not a silver bullet as some sites will not render great and some features will end up being hidden but for 95% of sites, dark mode is fine, including flyertalk.

in Chrome put chrome://flags in the location (no http://) and it's not in the usual options menu....Search for dark and you should find Force dark mode for web contentsChoose one of the options - I use Enabled with selective image inversionThe browser will need to restart to apply the change but it will preserve the state of all your pages.
Other browsers will probably have a similar option, google or bing should guide you...