To install in Firefox:
-
If it doesn't already exist, create a folder called
chrome
in your profile folder. The easiest way to find your profile folder is inabout: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. -
Put
userContent.css
above in thechrome
folder you just created. -
Load
about:config
in Firefox then settoolkit.legacyUserProfileCustomizations.stylesheets
to true. -
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.