Hi Khushboo,
traverse to following path on the server: /j2ee/cluster/server0/apps/sap.com/irj/servlet_jsp/irj/root/portalapps/com.sap.portal.design.portaldesigndata/themes/<THEME Name>/AFP/AFP_sf3.css.
Take the back up of existing AFP_sf3.css and make the below mentioned changes in that file.
Now search for the element: prtlPageConL.
Add a property to this element: Background-color: #FFFFFF;
Before change:
.prtlPageConL {padding: 0px !important;}
After change:
.prtlPageConL {Background-color:#FFFFFF;padding: 0px !important;}
the above one is an example for white background. You can add required color code there. Save this css file and upload on the same server path mentioned above by replacing the existing one.
Now, check whether the changes are reflected or not in the portal. If not, then restart the server.
Regards,
Khushboo