[Dark Theme]Fix for inconsistent Git Staging view bg colors

After improving the existing dark theme, Git staging view had side
effects wrt the back ground colors of whole view and also the Unstaged
Changes and Staged Changes section. This has been fixed with this change
by applying those dark background changes only to editor.

Author: mvm-sap

bundles/org.eclipse.ui.themes/css/e4-dark_linux.css

@@ -108,9 +108,9 @@ CTabFolder[style~='SWT.DOWN'][style~='SWT.BOTTOM'] {
     swt-selected-highlight-top: false;
 }
 
-.MPart Form Composite,
-.MPart Form Composite Tree,
-.MPartStack.active .MPart Form Composite Tree
+.Editor Form Composite,
+.Editor Form Composite Tree,
+.MPartStack.active .Editor Form Composite Tree
 {
 	background-color: #1E1F22;
 }

bundles/org.eclipse.ui.themes/css/e4-dark_mac.css

@@ -94,9 +94,9 @@ CTabFolder[style~='SWT.DOWN'][style~='SWT.BOTTOM'] {
     swt-selected-highlight-top: false;
 }
 
-.MPart Form Composite,
-.MPart Form Composite Tree,
-.MPartStack.active .MPart Form Composite Tree
+.Editor Form Composite,
+.Editor Form Composite Tree,
+.MPartStack.active .Editor Form Composite Tree
 {
 	background-color: #1E1F22;
 }

bundles/org.eclipse.ui.themes/css/e4-dark_win.css

@@ -176,9 +176,9 @@ CTabFolder[style~='SWT.DOWN'][style~='SWT.BOTTOM'] {
     swt-selected-highlight-top: false;
 }
 
-.MPart Form Composite,
-.MPart Form Composite Tree,
-.MPartStack.active .MPart Form Composite Tree
+.Editor Form Composite,
+.Editor Form Composite Tree,
+.MPartStack.active .Editor Form Composite Tree
 {
 	background-color: #1E1F22;
 }