Dark scrollbar for Firefox 57. Tested on Windows 10. Requires https://gist.github.com/Sporif/db6b3440fba0b1bcf5477afacf93f875

dark-scrollbar.as.css

@media (prefers-color-scheme: dark) {
	scrollbar, scrollbar *, scrollcorner {
		-moz-appearance: none !important;
		--scrollbar-width: 10px;
	    --scrollbar-height: var(--scrollbar-width);
	}

	scrollbar, scrollcorner  {
	    background: #282828 !important;
	}
	scrollbar[orient="vertical"] {
	    width: var(--scrollbar-width) !important;
	    min-width: var(--scrollbar-width) !important;
	}
	scrollbar[orient="horizontal"] {
	    height: var(--scrollbar-height) !important;
	    min-height: var(--scrollbar-height) !important;
	}

	scrollbar thumb {
		background: #646464 !important;
	}
	scrollbar thumb:hover,
	scrollbar thumb:active {
		background: #00ACED !important;
	}

	scrollbar:hover scrollbarbutton {
	    background-color: #444444 !important;
	}
	scrollbar scrollbarbutton:hover {
	    background: #646464 !important;
	}
	scrollbar scrollbarbutton:active {
	    background: #00ACED !important;
	}
}