Try a white sidebar

This PR extracts an idea from #9784, a white sidebar.

This looks a little jarring at first, but can grow on you. Because it's sort of a jarring change, I think it needs to be tested in a PR, rather than be discussed. That way people can actually _feel_ it, rather than look at screenshots.

What do you think?

edit-post/components/sidebar/style.scss

@@ -6,7 +6,7 @@
 	bottom: 0;
 	width: $sidebar-width;
 	border-left: $border-width solid $light-gray-500;
-	background: $light-gray-300;
+	background: $white;
 	color: $dark-gray-500;
 	height: 100vh;
 	overflow: hidden;

packages/components/src/panel/style.scss

@@ -34,7 +34,6 @@
 	display: flex;
 	justify-content: space-between;
 	align-items: center;
-	background: $light-gray-300;
 	padding: 0 $panel-padding;
 	height: 50px;
 	border-top: $border-width solid $light-gray-500;