My phone is always on dark mode and so is the browser aswell... the browser theme has always worked for me anyways.
Have you ever thought about reducing the brightness of your screen or activating Android's night mode?
I say this because these are useful solutions to avoid having to use extra apps or scripts.
I do that quite alot plus adding a privacy screen guard which reduces the light a little bit just that the disadvantage is that it reduce visibilityin day(especially when ou try to view under the sun).
Hence I use a particular app to go pro especially in the night , it's called 'Blue Light Filter" ( incase anyone is interested, not an advert )... infact , sometimes I do change it to black and white theme using the same filter app..
Adding a screen guard to my mobile phone could help reduce the brightness, which is beneficial, especially at night. Although it might be less effective in sunny conditions, I think it's still a better option than not having one. I haven't tried any apps for this purpose yet, but I plan to search for some, as I often struggle to operate my phone in the dark.