Theming: Allow to apply theme palette globally

Change-Id: I709bd6b6c566e016731af1fb94ddaef6326ef485
Reviewed-by: Thorben Kroeger <thorbenkroeger@gmail.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Theming: allow to override base style

Change-Id: I48e2d03b83fccf3d6eb4b508bd77912dbdaea1d0
Reviewed-by: Thorben Kroeger <thorbenkroeger@gmail.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>