Customizer: Fix `margin-top` calculation for Themes section.

Ensure that the Themes section calculates its `margin-top` as a panel, since the section behaves as one. Fixes regression caused by [35231].

See #33567.



git-svn-id: https://develop.svn.wordpress.org/trunk@35251 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/customize-controls.js

@@ -975,6 +975,16 @@
 			}
 		},
 
+		/**
+		 * Recalculate the top margin.
+		 *
+		 * @since 4.4.0
+		 * @private
+		 */
+		_recalculateTopMargin: function() {
+			api.Panel.prototype._recalculateTopMargin.call( this );
+		},
+
 		/**
 		 * Render control's screenshot if the control comes into view.
 		 *