How to stop white desktop on my MacBook Air from coming on ?

Are you trying to display a different desktop image other than the solid color white one?

If so, right click a blank spot on the desktop, select Change Desktop Background and choose the color or image you want.

Why are you invoking Command + shift? What problem are you experiencing?

You're presenting little to no information necessary to help you: Writing an effective Apple Support Commun… - Apple Community

Please be more descriptive about your problem.