Fix style book background color. (#68088)

Co-authored-by: jasmussen <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

packages/edit-site/src/components/editor-canvas-container/style.scss

@@ -1,6 +1,10 @@
 .edit-site-editor-canvas-container {
 	height: 100%;
 
+	// This is the gray background color that's applied behind "isolation mode".
+	// The color normally comes from .editor-visual-editor, but that class is missing here.
+	background-color: $gray-300;
+
 	// Controls height of editor and editor canvas container (style book, global styles revisions previews etc.)
 	iframe {
 		display: block;