All right, so the easiest way is to add some custom CSS. I prefer to do it by using the ‘custom css’ feature in Jetpack, but you can also look for .sidebar .widget in the theme’s CSS file and modify it directly.
.sidebar .widget {
background-color: #FFFFFF;
box-shadow: 0 3px 6px rgba(0, 0, 0, 0.2);
padding: 10px;
}
I added a little padding and the same shadow you have in the main content area. Just remove the box-shadow line if you don’t want it.