Hi All,

This is just some tweaks to the styling in ox-html that I think may
appeal (and prevent ridiculously long lines on non-small displays, which
are an issue for legibility).

I also took the opportunity to remove the (obsolete) CDATA strings and
make the CSS more consistently formatted. If you don't want this to
get its own commit, please just squash it.

Style changes:
- Restrict max content width, and centre
- tweak styling of source code blocks

I took some screenshots (1440p monitor, 120% zoom, Firefox).
Current: https://0x0.st/-iW9.png
This patch: https://0x0.st/-iWp.png

All the best,

Timothy.