Most good css site will have a preview shot of what their userChrome.css will look like. So what site are you using and did you contact them about the problem with the css? I use a site to make my firefox look like past firefox before FF57+ and have not issue with my userChrome.css that I installed.

STill no solution or fix months later. Im curious Mozilla, why is there a dark theme in Tools - Addons - Themes? Because it makes almost NOTHING dark. No default firefox pages, about: config, newtab, blank. Nothing is dark. Its an eye destroying bright white everywhere. How is that a 'dark theme'? You need to fix this crap. Users shouldnt have to write convoluted code files like this userContent.css in chrome folder in root profile folder.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); @-moz-document url("about:newtab"), url("about:blank")

{ body {background-color: #000000 !important;} } 

@-moz-document url(chrome://browser/content/browser.xul)

{ browser[type="content-primary"] {background-color: #000000 !important} }

Still this doesnt eliminate that glaring white flash right before every website loads. All that code does is make about newtab black...

Furthermore your ESR firefox version used in Tor Browser wont even allow me to modify any background color of any page, no config, about, newtab, nothing. This is extremely frustrating. Why do you force millions to suffer under bright white glare everywhere? No easy way to change colors to our preferences...Do you realize that damages peoples eyes? Like I dont get it. Why is this so hard to implement a simple color option for all pages?. I want everything fking inverted! I want black background with white text EVERYWHERE, on every website, firefox options, default pages. I need a solution. This is extremely not user friendly.

kingfitz said

Im just trying to have firefox have a dark theme, for everything. I want about:blank and about:newtab to be black background. why is this so complicated? I had to search for hours on this crap and write a custom css in chrome folder under default profile. I have it dark now but now it still flashes BRIGHT WHITE for a second before loading black. This shouldnt be so convoluted to fix. By the way, dark/black should be the default theme for every browser on planet earth. It would save millions from eyestrain/damage from harmful blue LED light from computer monitors and phone screens. Ive already selected default Dark Theme under themes. I tried the extension, Dark Background with Light Text. It now has interference from my custom css file I created, and wont work properly. Furthermore I cant change the new tab background on torbrowser either which is based off of firefox. Come on FF fix this nonsense, make customizing a theme EASY. its the simplest part of a website, the styling. And the fact that I click Dark Theme and everything should be dark, ITS NOT. about : blank and new tab are still GLARING WHITE! AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH

Here's a solution: https://stackoverflow.com/questions/57521737

~ Before: https://i.stack.imgur.com/TxRuK.png

~ After: https://i.stack.imgur.com/K0HOs.png

Enjoy.