With this change, we reduce the amount of inline CSS used
for EPUBs. Almost everything is now in the default EPUB
CSS (`data/epub.css`), which can be overridden either by
putting `epub.css` in the user data directory or by using
`--css` on the command line. Inline styles are only used
for syntax highlighting (which depends on the style specified,
and is only included on pages with highlighted code) and
for bibliography formatting (which can depend on the CSL
style, and is only used in the page containing the bibliography).
Note that, for compatibility with older readers, we don't
use flexbox to style `column/columns` divs by default, as
we do in HTML. Instead, we use an older method which only
works when there are two `column` divs inside a `columns`
div. If you need more than two columns and aren't worried
about support for older EPUB readers, you can modify the
default CSS (there is a comment in the CSS telling you what to do).
Closes#8379.
John MacFarlane