Because I have some light sensitivity issues, I use browser extensions including Dark Reader and Midnight Lizard to enforce my own 'dark mode' across the web.
You can also use extensions like that to set the contrast to a more comfortable level on websites that are already dark.
I highly recommend this if you have light sensitivity issues like me.
Also note that when the contrast on a page is higher, you can generally get away with lower brightness. This is pretty convenient on phones, and probably more necessary as well since on a phone you're more likely to have an OLED screen that really surfaces extreme contrast like white on black.
There are some great web extensions for a lot of things. I don't use any of them because most of them require permissions to read data across all sites, which makes sense for them to work; but I'm not using any of them.
Fair enough. I only use long-lived, open-source browser extensions for that kind of global restyling. But of course there's still a risk that they could be compromised somehow.