What tool/extension is being used to apply the custom stylesheets?
For example if you have .margin-width CSS class defined it will work on any page that has .margin-width CSS class, not just desired page (like Facebook).
Here's the MDN page detailing it and the various functions:
https://chrome.google.com/webstore/detail/stylish/fjnbnpbmke...
Chromium: ~/.config/chromium/Default/User\ StyleSheets/Custom.css
Chrome: ~/.config/google-chrome/Default/User\ StyleSheets/Custom.css Other browsers: Don’t know
https://chrome.google.com/webstore/detail/stylebot/oiaejidbm...
I know I could Google to find out, but what about the general Facebook user? They won't know what CSS is.