Fix default highlighting (alshedivat#931)

The default setting `media="none"` disables code highlighting. This
works OK when we have `enable_darkmode: true` in `config.yml` as the
code handling the dark/light theme switching takes care of setting
`media=""` for the preferred theme.
However, setting `enable_darkmode: false` prevents importing `theme.js`
which in turn prevents said change thus the default highlighting stays
disabled.
Setting `media=""` by default enables the default highlighting when
`enable_darkmode` is false.

Fixes alshedivat#929