Customise Firefox extension using chrome/userChrome.css

Question

I have foxclocks installed "{d37dc5d0-431d-44e5-8c91-49419370caa1}.xpi"

I have enabled use of chrome/userChrome.css and have hidden menu items so I know it is working.

I like to hide the "closer" in fox clocks but how do I reference it in chrome/userChrome.css

I tried

#closer { display: none !important } 

but didnt work?

See image below

Answer 1

Works by doing the same with userContent.css and turning on

toolkit.legacyUserProfileCustomizations.stylesheets to true 

in about:config.