@david
Documentation notebooks use a built-in stylesheet and they have disabled editing it using the standard mechanism, grrr... but you can likely go edit the file directly, likely located at
FileNameJoin[{$InstallationDirectory,"SystemFiles","FrontEnd","StyleSheets","Wolfram","reference.nb"}]
which I learned by running the following in a documentation notebook ![enter image description here](https://community.wolfram.com//c/portal/getImageAttachment?filename=nb.png&userId=26311)