The background color of a chart theme is not shown in de iReport viewer (in subreports).
In the example below in the main report the background colors are not there in the Internal Viewer. They are there in the PDF preview.
Reproducable with the samples provided with iReport itselves, see below.
Sometimes the background colors are there but when the same report is executed from an other report as subreport, the background colors lack. Also then in the PDF preview they are there.