fix editor background regression when all editors are closed

src/vs/workbench/browser/parts/editor/media/sidebyside.css

@@ -5,11 +5,11 @@
 
 /* Editor Container Styles */
 
-.vs .monaco-workbench .editor > .content {
+.vs .monaco-workbench .editor:not(.empty) > .content {
 	background-color: #F3F3F3;
 }
 
-.vs-dark .monaco-workbench .editor > .content {
+.vs-dark .monaco-workbench .editor:not(.empty) > .content {
 	background-color: #252526;
 }
 

src/vs/workbench/browser/workbench.ts

@@ -808,7 +808,7 @@ export class Workbench implements IPartService {
 	private createEditorPart(): void {
 		let editorContainer = $(this.workbench)
 			.div({
-				'class': ['part', 'editor'],
+				'class': ['part', 'editor', 'monaco-editor-background'],
 				id: Identifiers.EDITOR_PART,
 				role: 'main'
 			});