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

dark-scrollbar.as.css

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;
}

@bunnyboothe What do you mean you did everything again? I'm on FF 68.0 and it's still not working for me. I just have default scrollbars right now. Please advise.

EDIT: I made this fix in the userChrome.css and it's all working now :)

@e-t-l can you explain what you have done exactly? I'm struggling with FF 68 yet. :(

EDIT: I had to add to include Sporif. But scrollbar arrows disappeared. :(

Okay it worked with sporif's userchrome.css code? I believe they have some info in the comments of that page regarding how to activate the scrollbar arrows. Personally I leave them deactivated so I don't know offhand.

@Sporif I noticed these scrollbars get larger when the webpage is zoomed in. Is there a way to disable this scaling?

@e-t-l They get larger because he is using pixel to determine the size instead of view-width.

Edit: Try this one.