fdo#75040 fix application of help's stylesheet

it was regression from 73e3aafa990168aa532fa7b81fc4de8f455b10e1

Change-Id: I059f08b12bb7847472946f3b2a669015ec437be5
(cherry picked from commit 2e3969cb8bb235fed483ce49ddee5ea731376893)
Reviewed-on: https://gerrit.libreoffice.org/8609
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>