firefox-profile.css

/* In Profiles/<your profile/chrome/userChrome.css */
.browserContainer {
	background-color: #333 !important;
}

/* In userContent.css */
@-moz-document url-prefix(about:blank), url-prefix(about:newtab) {

	html:not(#ublock0-epicker), html:not(#ublock0-epicker) body, #newtab-customize-overlay {
		background: #3d3d3d !important;
	}

	@-moz-document url(about:newtab),
	url(about:home) {

		body{
			background-color:#3d3d3d!important;
			color:#D1D1D1!important;
		}

		.card,
		.prefs-pane .sidebar {
			background-color:#202020!important;
		}

		#snippets,
		.section-title,
		.snippetContainer {
			color: #AEAEAE!important;
		}

		.contentSearchSuggestionTable,
		.contentSearchSuggestionsList {
			color:black!important;
		}

		.display-item {
			color: #D1D1D1!important;
		}

		#aboutMozilla::before {
			filter:invert(100%)!important;
		}

		#searchText{
			background:#474749!important;
			box-shadow: none;
		}

		#searchSubmit,
		#launcher {
			display:none!important;
		}
	}
}