One missing thing is the support in calc to save the Theme into
the ODS document and read that back. The theme element is added as
a child element to the office:styles - the same as it already is
added in Writer.
Also adds "Theme" property as a top level document property
so it is possible to get and set the theme in xmloff.
Also tests have been added to cover this usecases.
Change-Id: Ic214ff5e945b77d50e6c881def9d49509560a0e0
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/156363
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
