#' @param extra_css A named list of CSS definitions each containing a named list
#'   of CSS property-value pairs, i.e.
#'   \code{list(".class-id" = list("css-property" = "value"))}
#' @param extra_fonts A list of additional fonts to import, each list element
#'   can be either a URL as a character string or a call to
#'   \code{\link{google_fonts}()}