Like the title says^^

Applies on every website & changes as much as possible, also elements like scrollbars, buttons, cursors or the right-click menu to fit with the dark style (best result after Firefox restart).

I'm sorry if it doesn't work good on your website but it's just not possible to make it look good everywhere. But I try my best to do so :)

Easy customizeable with your own background. Just edit the style and paste the image-url.
To use an background-image from your hard-drive, drag the image into the browser-window and copy the path from the adressbar and paste it at the url-part. It should look like this: url("file:///C:/Wallpaper/Wood.jpg")

If your scrolling is slow or you just want a simple black background, remove the url-line.
If you want the background to be fixed, remove the comments on "fixed".

Now you can also exclude websites! Just edit the style and replace/add 'www.your.sites.here.com' with your desired ones.