In a theme panel the user can change / replace the styles font
and colors used in some predefined style elements (for example
Heading). This is "fake" theme support and not the same thing that
MSO uses, but it still can be useful for the users to change the
appearance of the document. This is the initial commit of the work
and will be much extended. Currently it is only available when
experimental mode is enabled.

Change-Id: I8d34ce87d21975ec6020ac45ecaebb0701b63b2a