Their official name is “Firefox Themes” nowadays.
While I was there, tweak some strings.

Change-Id: I2505df2fb810fa7de51d28f5704bc1178671c3ac
Reviewed-on: https://gerrit.libreoffice.org/11483Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>