Dark Mode

userChrome.css:

/* Remove New Tab Flash */
@-moz-document url(chrome://browser/content/browser.xul)
{
  browser[type="content-primary"], tabbrowser tabpanels, #appcontent > #content
  {
	background: #323234 !important;
  }
}
#tabbrowser-tabpanels {
  background: var(--toolbar-bgcolor) !important;
}

userContent.css:

@-moz-document url(about:blank),
			   url(about:newtab)
{
  #newtab-window,
  html
  {
	background: #323234;
  }
}

@-moz-document url-prefix(about:blank), url-prefix(about:newtab), url(moz-extension://db85f1de-ba17-4b39-8a06-7e124677ffd6/html/newtab.html) { html:not(#ublock0-epicker), html:not(#ublock0-epicker) body, #newtab-customize-overlay { background: #323234 !important; } }

The last line in userContent.css is to darken the page before the loading of new tab override. To get the weird id looking string, go to the settings of new tab override and look at the url.