Dark Background and Light Text
Can you imagine what a web browser extension called "Dark Background and Light Text" does? =)
I'll tell you what it does: amazing things!
I've used similar extensions in the past to "counteract" the ultrawhiteness that plagues the WWW, but this one is much, much more capable. You can have per-site settings: disable for some sites, use a different "coloring method" (Stylesheet processor, simple CSS, invert) for others... and it's a WebExtension, so it can be used in Firefox 57+, and other browsers =)