Hi There,
Thanks for writing in! Please navigate to Appearance > Customize, then on Customizer under the Layout and Design look for the Background Color option and set it to white.
The greyish background color of the landmark and sidebar is actually a (default) background pattern, if you have not reset it. You can remove that by setting a temporary Background Pattern, then if you see the background pattern on the preview just simple remove it. And the white background color that you set should immediately visible.

Provide us the site URL if this does not work.
Hope it helps, Cheers!