Hi Anand,
First thing is that BackgroundColor is available for modification from the first tab in Theme Designer. No need to change it via CSS. Why make it harder on yourself?
Second, there is a whole section in the Fiori Landing page with links to lots of Theme Designer documents and guides. I would suggest that you start there and just play around with Theme Designer a bit to see what LESS parameter controls the area where you want the color to change.
Regards, Mike
SAP Customer Experience Group - CEG