Comment 3 for bug 875545

The problem is, as explained, that userstyles themselves are implemented via an extension, and at present there's no policy to prevent extensions from leaking private-browsing sessions' data to disk. Unless/until that changes, it's unlikely that extensions will become available in private-browsing mode.

A workaround for the present is to use the user-stylesheet-uri setting in the config file; it's a uri that points to a file to be loaded as CSS for all pages, like a global userstyle that doesn't require the extension. For example, add the line "user-stylesheet-uri=file:///home/youruser/midori.css" to ~/.config/midori/config if your css is in ~/midori.css.