Google Chrome seems to have options for websites that do not offer a dark mode or dark colour scheme.

In Firefox you can go to settings->fonts &colors. Here you can choose colours for fonts and background and then select 'Apply always' or something like that. (I am working with the german version.) I tried this with a dark grey and it replaces both the white and the sand colours on GitP. This does apply to all websites and does not exactly look pretty but might be a workaround for you.