Hi there,
You can set the background color of the <body>
tag to white or #ffffff on Appearance > Customize > Color.
You can then remove the box shadow on then CSS:
.site-wrapper.grid-container {
max-width: 1600px;
background: #FFFFFF;
box-shadow: 0px 0px 10px 0px rgba(0,0,0,0.3);
}
This CSS is found on Appearance > Customize > Additional CSS area.
Note: You can remove this whole CSS entirely if you also wish to remove the width restriction. Currently it is set to 1600px.