By default, pandoc will include some basic styling contained in its
epub.css data file. (To see this, use
pandoc --print-default-data-file epub.css.) To use a
different CSS file, just use the --css command line option.
A few inline styles are defined in addition; these are essential for
correct formatting of pandoc’s HTML output.
The document-css variable may be set if the more
opinionated styling of pandoc’s default HTML templates is desired (and
in that case the variables defined in Variables for HTML may
be used to fine-tune the style).