Hello,

I just noticed that using the command-line -c (respectively, -H) option overrides any css: (respectively, header-includes:) keys in the YAML metadata sections.

This is a bit surprising, since the options themselves are otherwise cumulative (one can specify multiple -c and -H options, and they get all used), and makes it much harder to have both common and file-specific CSS and header includes (in this sense, this bug is related to #3115).

I believe that, to avoid violating the principle of least surprise, the -c and -H options should be cumulative with the file metatada (and specifically added after the internal ones). OTOH, such a change in behavior would break backwards compatibility, so maybe a way to specify additional CSS and header-includes from the command line should be given?

(If the replacing behavior is preserved, it would also be a nice idea to document it in the manual.)