A dark mode extension I'm using on Firefox for Android works as an css injector. One would only have to configure a custom stylesheet instead of a dark theme.

https://addons.mozilla.org/en-US/firefox/addon/dark-mode-web...

uBlock Origin, the content blocking extension I'm using on Firefox for Android, can inject css rules as well.

https://addons.mozilla.org/en-US/firefox/addon/ublock-origin...