Suggestion: Add "toolkit.legacyUserProfileCustomizations.stylesheets" step to README

[m4729316-commits]

Hi there,

Thanks for creating this awesome tool! It works perfectly.

However, I noticed that the README doesn't mention that users need to enable custom stylesheets in Firefox for the theme to apply. Without this, the userChrome.css changes won't be loaded by the browser, atleast in my case.

It might be helpful for less experienced users to add this quick step to the installation guide:

1. Open Firefox and type "about:config" in the address bar.
2. Search for toolkit.legacyUserProfileCustomizations.stylesheets.
3. Set its value to "true".
4. Restart Firefox.

Thanks again for your work! :)