See this tweet for how I use this.
To install in Firefox:
If it doesn't already exist, create a folder called
chromein your profile folder. The easiest way to find your profile folder is in
about:profiles(there's a "Show in Finder" link for the folder on macOS). Be sure to choose the right profile is you have more than one.
userContent.cssabove in the
chromefolder you just created.
about:configin Firefox then set
Quit and reopen Firefox, just for luck.
Then to use the stylesheet, activate Reader View (cmd-alt-R on macOS, other there's a button in the location bar), and then print the page. The printout will look roughly like an NYRB page.