It was adapted from another rule, so some of the selectors in "Page Text & Background" may be crufty.

Tested on Firefox with Pi-hole Version v2.11 & Web Interface Version v2.3.

***IMPORTANT***
Once you install this userstyle, edit the @moz-document section:

Replace

@-moz-document url-prefix(http://), url-prefix(https://), url-prefix(ftp://), url-prefix(file://)

With

@-moz-document domain("pi.hole")

If you use your IP address instead of 'pi.hole', replace 'pi.hole' with your IP address.

Userstyles.org doesn't recognize pi.hole as a valid domain, so it will not allow me to save that change directly to the userstyle. I apologize for the inconvenience.