Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 33583)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -685,7 +685,6 @@
 		 */
 		ready: function () {
 			var section = this;
-			section.overlay = section.container.find( '.theme-overlay' );
 			section.template = wp.template( 'customize-themes-details-view' );
 
 			// Bind global keyboard events.
@@ -733,6 +732,7 @@
 		 */
 		attachEvents: function () {
 			var section = this;
+			section.overlay = section.container.find( '.theme-overlay' );
 
 			// Expand/Collapse section/panel.
 			section.container.find( '.change-theme, .customize-theme' ).on( 'click keydown', function( event ) {
@@ -806,6 +806,11 @@
 					}
 				});
 			});
+
+			// Contain tabbing within the Theme details modal.
+			section.overlay.on( 'keydown', function( event ) {
+				section.containFocus( event );
+			});
 		},
 
 		/**
@@ -1025,6 +1030,8 @@
 			$( 'body' ).addClass( 'modal-open' );
 			section.containFocus( section.overlay );
 			section.updateLimits();
+			// Get and update the tabbable elements for each theme details modal. Keep this after updateLimits().
+			section.getTabbables( section.overlay );
 			callback();
 		},
 
@@ -1044,30 +1051,44 @@
 		 *
 		 * @since 4.2.0
 		 */
-		containFocus: function( el ) {
-			var tabbables;
+		containFocus: function( event ) {
 
-			el.on( 'keydown', function( event ) {
+			// Return if it's not the tab key or if it's navigation with arrows.
+			if ( 9 !== event.which || 37 === event.which || 39 === event.which ) {
+				return;
+			}
 
-				// Return if it's not the tab key
-				// When navigating with prev/next focus is already handled
-				if ( 9 !== event.keyCode ) {
-					return;
-				}
+			// Keep focus within Theme details modal.
+			if ( this.tabbables.last()[0] === event.target && ! event.shiftKey ) {
+				// Prevent default focusing.
+				event.preventDefault();
+				this.tabbables.first().focus();
+			} else if ( this.tabbables.first()[0] === event.target && event.shiftKey ) {
+				// Prevent default focusing.
+				event.preventDefault();
+				this.tabbables.last().focus();
+			}
+		},
 
-				// uses jQuery UI to get the tabbable elements
-				tabbables = $( ':tabbable', el );
+		/**
+		 * Store the tabbable elements within the Theme details modal.
+		 *
+		 * Initially set to an empty jQuery object, gets updated with collection returned by getTabbables().
+		 *
+		 * @since 4.4.0
+		 */
+		tabbables: $(),
 
-				// Keep focus within the overlay
-				if ( tabbables.last()[0] === event.target && ! event.shiftKey ) {
-					tabbables.first().focus();
-					return false;
-				} else if ( tabbables.first()[0] === event.target && event.shiftKey ) {
-					tabbables.last().focus();
-					return false;
-				}
-			});
-		}
+		/**
+		 * Get the tabbable elements within the Theme details modal.
+		 *
+		 * Uses jQuery UI to get the tabbable elements.
+		 *
+		 * @since 4.4.0
+		 */
+		getTabbables: function( el ) {
+			this.tabbables = $( ':tabbable', el );
+ 		}
 	});
 
 	/**