Firefox CSS to style scrollbars to match Developer's theme. Use with userChrome.css

scrollbars.as.css

/*
scrollbars.as.css
based on https://userstyles.org/styles/128059/firefox-minimal-scrollbars

On Windows, save this file as: %APPDATA%\Mozilla\Firefox\Profiles ... /chrome/scrollbars.as.css
via: https://gist.github.com/Sporif/db6b3440fba0b1bcf5477afacf93f875

paste in userChrome.css in the /chrome folder mentioned above:
	#PanelUI-menu-button {
	    -moz-binding: url(data:text/xml;base64,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);
	}
*/

scrollbar {
	z-index: 999;
	position: relative !important;
	transition: all 0s !important; 
}

scrollbar,
scrollbar * {
	-moz-appearance: none !important;
	margin: 0px !important;
	padding: 0px !important;
	border: 0px !important;
	box-shadow: none !important;

}

:not(select)>scrollbar[orient="vertical"],
html>scrollbar[orient="vertical"] {
	-moz-margin-start: -12px !important;
	max-width: 12px !important;
	min-width: 12px !important;

	background: linear-gradient( rgba(0,0,0,0), rgba(0,0,0,0) );
	background-size: 1px 1px;
	background-repeat: repeat-y !important;
	background-position: 50% 0% !important;
	cursor: row-resize;
}

scrollbar[orient="horizontal"] {
	margin-top: -12px !important;
	max-height: 12px !important;
	min-height: 12px !important;
  
	background: linear-gradient( rgba(0,0,0,0), rgba(0,0,0,0) );
	background-size: 1px 1px;
	background-repeat: repeat-x;
	background-position: 0% 50%;
	cursor: col-resize;
}

scrollbar thumb[orient="vertical"] {
	min-height: 24px !important;
	width: 12px !important;
	min-width: 12px !important;
	max-width: 12px !important;
}

scrollbar thumb[orient="horizontal"] {
	min-width: 24px !important;
	height: 12px !important;
	min-height: 12px !important;
	max-height: 12px !important;
}

scrollbar thumb {
	border-radius: 6px !important;
	background: transparent !important;
	border: 2px solid rgba(0,255,255,0) !important;
	box-shadow: 0 0 0 8px #272b35 inset !important;
	transition: all 0s !important;
	opacity: 0.1 !important; 
}

scrollbar:hover thumb,
scrollbar thumb:active {
	box-shadow: 0 0 0 8px #cd23b9 inset !important;
	opacity: 1 !important;
}

/* no buttons */
scrollbar scrollbarbutton{
	min-height: 0px !important;
	min-width: 0px !important;
	max-height: 12px !important;
	max-width: 12px !important;
	height: 0px !important;
	width: 0px !important;
}

/* autohide */
/*scrollbar:not(:hover) { 
	opacity: 0 !important; 
	transition: all .5s !important; 
}*/

See https://gist.github.com/Sporif/db6b3440fba0b1bcf5477afacf93f875 for a simpler solution that doesn't require userChrome.js in Firefox 57+ and https://github.com/Izheil/Firefox-57-full-dark-theme-with-scrollbars for multiple other stylesheets.