You can switch to dark mode and affect the whole UI, which you may not like. If you do not want to affect the rest of your browsing experience, [https://support.mozilla.org/cs/questions/1305573 here] is a way to do this (I have done it as well) but it requires your concentration for about 10 minutes as you need to follow instructions carefully. Make sure to [https://www.userchrome.org/how-create-userchrome-css.html set up userchrome.css correctly] as this is not a user-friendly nor straightforward proccess. Reply in case of any issues.