Main theme
Install Compact Dark theme in Appearance

All websites dark theme
https://addons.mozilla.org/en-GB/firefox/addon/dark-background-light-text

New tab whitescreen issue:
1. Options -> Colours in FireFox
   Change background to black and foreground to white
   Override set to never
   This fixes initial opening up Firefox background white

2. Install customize about:newtab 
   https://addons.mozilla.org/en-US/firefox/addon/customize-about-newtab/?src=search
   Change background color to black, and uncheck all "Show xxx"
   This fixes new tabs being white

