org-html-htmlize-output-type

org-html-htmlize-output-type is a variable defined in `ox-html.el'.
Its value is

inline-css

Documentation:
Output type to be used by htmlize when formatting code snippets.
Choices are `css', to export the CSS selectors only, or `inline-css', to
export the CSS attribute values inline in the HTML. We use as default
`inline-css', in order to make the resulting HTML self-containing.

However, this will fail when using Emacs in batch mode for export, because
then no rich font definitions are in place. It will also not be good if
people with different Emacs setup contribute HTML files to a website,
because the fonts will represent the individual setups. In these cases,
it is much better to let Org/Htmlize assign classes only, and to use
a style file to define the look of these classes.
To get a start for your css file, start Emacs session and make sure that
all the faces you are interested in are defined, for example by loading files
in all modes you want. Then, use the command
M-x org-html-htmlize-generate-css to extract class definitions.

You can customize this variable.