This line of JS will make the text in every paragraph become black:

document.querySelectorAll('p').forEach(elem => {elem.style.color = '#000'})

You can turn this into a bookmarklet or run it from the console(necessary on sites like github.com which do not allow code running from bookmarklets).

Of course, it will not fix all your text color problems. To cover more elements like lists and headings, you'll need to add them to `querySelectorAll`.

document.querySelectorAll('p, ul, ol, h1, h2, h3, h4').forEach(elem => {elem.style.color = '#333'})

`#333` is what Firefox Reader Mode uses because it's less heavy than full black (#000).