Firefox White Page Loading -> Dark page

How-to_firefox-white-loading-switch-to-dark.css

/* 
Go to about:config, set toolkit.legacyUserProfileCustomizations.stylesheets to true.
Create a userChrome.css file according to these instructions: https://www.userchrome.org/how-create-userchrome-css.html
Paste the following into userChrome.css 
*/

/*No white flash on new tab opening in dark theme */
#browser vbox#appcontent tabbrowser, #content, #tabbrowser-tabpanels, browser[type=content-primary], browser[type=content] > html { background: #222 !important; }

/* 
You may also need to create userContent.css and paste this into it:
*/

/* dark blank tab */
u/-moz-document url(about:blank), url(about:newtab) {
	#newtab-window, html:not(#ublock0-epicker) {
		background: #222 !important;
	}
}