For more completeness, type in about:config in the url bar and change "layout.css.scrollbar-color.enabled" to "false" and use the Dark Scroll extension. For even better, go to https://pastebin.com/raw/1RasKHcH and save it as userChrome.css in a folder called "chrome" and store it in your firefox profile directory, close and restart Firefox and context menu, urlhover and bookmark dropdowns should be themed also.
Will be updated for dark context menu, dropdowns and such once Mozilla fixes the related bug already filed.
