Browsers _should_ have been set up to allow per-site toggling of the dark mode CSS preference, much like how they allow per-site zooming.
Configuring the whole browser to have a light theme is the wrong solution - some websites look better in dark mode and some look better in light mode. Also, the browser setting also affects the UI of the browser itself, not just website contents.
These are of course solvable problems, but the most obvious and trivial way to handle this is just to store an extra flag per-website in the same place as the zoom preference.
Is that persistent? As in, if I select a style sheet for a website, will it use that same style sheet if I reload the website or navigate back at a later date? Browsers do this all the time with zoom levels, but it would be really useful for much more than that.
Configuring the whole browser to have a light theme is the wrong solution - some websites look better in dark mode and some look better in light mode. Also, the browser setting also affects the UI of the browser itself, not just website contents.
These are of course solvable problems, but the most obvious and trivial way to handle this is just to store an extra flag per-website in the same place as the zoom preference.