I made a dark theme for the CL cookbook. If you have the Stylus browser extension installed, you can add this:
@-moz-document domain("lispcookbook.github.io") {
html {
filter: invert(100%);
}

div[style="height: 27px;"] {
filter: invert(100%); /* double invert to keep it black */
}

img {
filter: invert(100%);
}

video {
filter: invert(100%);
}
}

The CSS is really generic, and it can work for a lot of other sites.

https://lispcookbook.github.io/cl-cookbook/web-scraping.html
https://github.com/openstyles/stylus