Try adding some path. info to the file name (ie --extra-css "./custom_markdown.css" or possibly --extra-css "/the/path/where/the/file/exists/custom_markdown.css"
From the help
Quote:
--extra-css
Either the path to a CSS stylesheet or raw CSS. This CSS will be appended to the style rules from the source file, so it can be used to override those rules.
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