From another comment, I see you are on Firefox, but maybe these two plugins are available there too (or similar ones). First, there is "dark reader", which als

From another comment, I see you are on Firefox, but maybe these two plugins are available there too (or similar ones). First, there is "dark reader", which also has a light mode. You can change things like brightness and contrast and this should make it possible to get the text to black on basically all websites by default. I say should, because I didn't try the light mode myself, just noticed it's there. Secondly, a more manual and fine-grained option would be "Stylus" which lets you manage CSS snippets per URL (or part of URL) and injects them on pageload for you. I do use both of them myself and wouldn't want to miss them. Of course they both need "read and write all content on all visited websites"-privileges, so that is something to consider as well.