Change color of loading page in Safari When a webpage is loading in Safari, a white screen appears, be it Facebook, Gmail or any other website. If it's contents haven't appeared, then the tab appears completely white (maybe my Internet is way too slow but it takes a few seconds with almost every page, that's why it is annoying). 

Is there a way to override this color so that it appears gray/black until the page is loaded (regardless if the webpage has a white background)?

I am running macOS Mojave 10.14.4.
It is not possible to customize the color of loading page in Safari. However you can enable dark mode by going to System Preferences app → General → Appearance to change from white to dark gray color for the loading view.

The dark mode however will get applied system wide and not just to Safari.