Graph output in SAS Report and HTML does not honor the stylesheet settings. Instead, the graph appearance is set when the graph is generated on the SAS server.
You can influence the graph appearance by using color option on the graph (set the background color in an EG graph task, or use the GOPTIONS CBACK= option, for example). If you want to customize the style definition, you can create a custom ODS style template using PROC TEMPLATE. More information about ODS and PROC TEMPLATE is in the Base SAS Focus Area (http://support.sas.com/rnd/base/index.html), or you could post a question to the Base SAS/ODS discussion forum.
Chris