The title is self-descriptive. For the love of all that is holy, do it for the sake of our eyes.
I agree it should be an option, just like you have a Dark Mode in Youtube.
Until that (never) happens, you can use Stylus in Firefox or something else in Chrome. Dark themes for Unreal are here: https://userstyles.org/styles/browse…realengine.com
There’s already dark theme for the forum, documentation sites (hidden at the bottom of the pages), marketplace and blog.
Answer’s Hub, “global search” and Issues lacks dark theme yet. I guess it’s all scheduled.
Generally, I recommend Dark Reader for browser and turning it on for awfully white sites that don’t provide dark mode yet. It’s a blessing 
