doc/_themes: fine tune styling

doc/_themes: fine tune styling

* fix the "dl" (definition list) display in manpage. before this
  change, if an item in dl is composed of strings with different fonts,
  it is cluttered into a single string. after this change, they are
  separated with proper spacing as before. see the "WORKLOAD" section in
  https://docs.ceph.com/en/latest/man/8/ceph-syn/ and
  https://docs.ceph.com/en/octopus/man/8/ceph-syn/
* reduce the spacing in "ul" (unordered list) in table. before
  this change, we have large spacing at end of a list in table. after
  this change, the spacing is reduced. see
  https://docs.ceph.com/en/latest/start/hardware-recommendations/#ram
  https://docs.ceph.com/en/octopus/start/hardware-recommendations/#ram
* increase the spacing between two "ul"s in table. see
  https://docs.ceph.com/en/latest/start/hardware-recommendations/#Processor
  https://docs.ceph.com/en/octopus/start/hardware-recommendations/#ram

Signed-off-by: Kefu Chai <kchai@redhat.com>