I use the Stylebot extension for Firefox, and made a css script for the Unreal Forums and Documentation. It’s not perfect, but it gets the job done for me.
(I posted the css code in this reply, and while I haven’t checked it in Chrome, you can probably use the same css code for the Stylebot extension for Chrome as well.)
https://forums.unrealengine.com/t/need-a-bright-mode-for-the-documentation/1773134/7?u=shookypooks