Added missing step for live editing configuration (#206)

Author: list-jonas

content/docs/guides/live-editing.mdx

@@ -33,6 +33,7 @@ This Guide will help you customize the appearance of Zen Browser by live editing
   1. Open the `about:config` page. This page contains advanced settings for the browser.
   2. Search for `devtools.debugger.remote-enabled` and toggle it to `true`.
   3. Search for `devtools.chrome.enabled` and toggle it to `true`.
+  4. Search for `toolkit.legacyUserProfileCustomizations.stylesheets` and toggle it to `true`. This setting is required for `userChrome.css` to work.
 </Callout>
 
 1. In Zen Browser, press <KeyboardShortcut shortcut="Ctrl + Shift + Alt + I" /> to open the Developer Tools.