Scrivener 3 on Windows 10

Really banging my head here. From what I understand, compiling in epub is the only way to utilize custom css. However, for the life of me I can't get it to work no matter what I try.

Here's the process I'm using:

1. In main "Compile Overview" window, I've assigned a section layout to my "scenes". (I'm assigning "section text" to scenes)

2. In the "Compile Format Designer", under "Section Layouts" option, I scroll down to "Section Text" (which is what's assigned to my scenes). Under the "settings" tab, I have assigned the CSS class name mydiv

3. Now, I click the CSS option. I switch the dropdown to "Append Custom CSS Stylesheet". Beneath this is a textfield labeled "custom Stylesheet", which already has a lot of css in it (which I am able to edit). I add the following in there, just as a test: .mydiv p { font-family: courier; } EDIT: By the way, p. 553 of the user manual gives some clarity regarding the options in this dropdown: "use default css stylesheet" means it'll only use the css in the right pane (labeled default), "use custom css stylesheet" means only the the css in the left pane (labeled custom), and "append custom css stylesheet" means it will use both panes.

4. Click "Save" to close the designer.

Unfortunately, when I compile, it does not take my css.

Here's a few questions, that might highlight what I'm doing wrong:

1. Under the CSS option in the Compile Format Designer, am I supposed to check or uncheck "Create styles for paragraphs using custom formatting"? (I am unclear what this option means). I tried both, but it didn't appear to make a difference.

2. Under the Section Layouts option in the Compiler Format Designer, when I scroll to "Section Text" (the layout that's been applied to my scenes), in the "Formatting" tab, there is a dropdown at the bottom with the following options: "Text and notes use default paragraph formatting", "Text and notes use custom formatting", and "Text and notes use editor formatting". Which option should I be selecting here, if I want to get custom css to work? (I tried "Text and notes use custom formatting", but it still didn't work.)

3. In the css option, should I Be removing everything from the "custom stylesheet" other than my own css? I'm not even sure where this css is coming from. The other css seems related to headers; nothing that would override my custom p styling. see edit above.

4. Is there a way to have my own external css file, and just provide the filepath to this css file? (As user feedback - I think this would be far easier, more convenient, and more intuitive.)

5. Is there documentation specifically for this? I couldn't find any.

Really love the option to use custom css, but it doesn't seem clear how to utilize it. I've been working with css for well over a decade (though I'm FAR from a css expert), so don't worry about overwhelming me with terms, etc. Any help is appreciated.

EDIT: I was able to get it working... sort of... I am able to apply some css by going to the 'css' option, switching dropdown to 'use custom css stylesheet' and styling general elements (i.e. just style p elements, do not use classes). I am unable to use css classes as mentioned above. Also, the general p styling still gets overridden by some things in the program editor (for example font-style: normal is ignored and text will be italicized if there is italicized text in the main editor, even though I'm applying a layout to my section text (not 'as-s'). Not sure why..).