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

colorful-scrollbar.as.css

scrollbar, scrollbar *:not(scrollbarbutton), scrollcorner {
	-moz-appearance: none !important;
	--scrollbar-width: 15px;
    --scrollbar-height: var(--scrollbar-width);
}

scrollbar, scrollcorner  {
    background: #fff !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[orient="horizontal"] thumb {
	background: linear-gradient(to bottom, rgb(255,255,125) 0%,rgb(254,215,76) 47%,rgb(248,194,0) 53%,rgb(253,230,145) 86%,rgb(248,194,0) 100%) !important;
}

scrollbar[orient="horizontal"] thumb:hover,
scrollbar[orient="horizontal"] thumb:active {
	background: linear-gradient(to bottom, rgb(255,255,87) 0%,rgb(253,208,45) 47%,rgb(224,175,0) 53%,rgb(252,221,106) 86%,rgb(224,175,0) 100%) !important;
}

scrollbar thumb {
	background: linear-gradient(to right, rgb(255,255,125) 0%,rgb(254,215,76) 47%,rgb(248,194,0) 53%,rgb(253,230,145) 86%,rgb(248,194,0) 100%) !important;
	border: 1px solid lightblue;
	border-radius: 15px;
}
scrollbar thumb:hover,
scrollbar thumb:active {
	background: linear-gradient(to right, rgb(255,255,87) 0%,rgb(253,208,45) 47%,rgb(224,175,0) 53%,rgb(252,221,106) 86%,rgb(224,175,0) 100%) !important;
	border-color: #0494c3;
}

scrollbar scrollbarbutton {
	border: 1px solid lightblue;
	color: lightblue;
	background-color: blue;
	height: 20px;
}
scrollbar scrollbarbutton[type="increment"] {
	-moz-appearance: none !important;
}
/*
scrollbar:hover scrollbarbutton {
    background-color: #444444 !important;
}
scrollbar scrollbarbutton:hover {
    background: #646464 !important;
}
scrollbar scrollbarbutton:active {
    background: #00ACED !important;
}*/