I use Scrivener 3 for windows. I am on the latest (3.1.4 and windows 11).
The user manual, 24.6, talks of a “CSS pane” where changes to the generated css can be applied but I cannot find this “pane”.
I do have a minor change I want to apply to the css generated for an ebook format but I cannot find this “Css pane”. Currently I am having to use Sigil to edit the css element but I would prefer to do it in Scrivener so it gets compiled right in the first place, but cannot see how to do this within Scrivener. Any suggestion much appreciated.
First, compile to ePub2 , ePub3, or Mobi at the top of the Compile Overview.
Second, double click the eBook compile Format in the left column of the Compile Overview window.
Copy and Edit the Format when asked.
The Compile Format Designer opens, check the title of the window.
You’ll find the CSS pane halfway the menu on the left of that screen.
Note that when done with compiling your e-book, you’ll still need Sigil, so don’t uninstall that software. You may want to embed fonts or correct deep lists in the output from Scrivener.
Great. I would never have worked that out… Thanks.
Not planning to uninstall Sigil, although Calibre also has an ebook editor which I also use on occasion. 
Yep. I use Sigil and Calibre both too. 
I’m convinced that’s a Usability issue that should be addressed, but for now it remains as it is.
Actually there are already five different ways to reach that screen, but they’re mostly invisible.