Accessibility: Improve the Customizer and Theme Installer initial focus.

The Customizer and Theme Installer open in full overlays that need to receive 
focus. Also, keyboard navigation should be constrained within the overlays. Using
CSS `visibility` to hide all the content except the overlays, makes them the only
available and focusable content and allows browsers to handle focus natively.

See #29158.
Fixes #33228, #27705.


git-svn-id: https://develop.svn.wordpress.org/trunk@38520 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/themes.css

@@ -1182,6 +1182,8 @@ div#custom-background-image img {
 
 body.full-overlay-active {
 	overflow: hidden;
+	/* Hide all the content, the Customizer overlay is then made visible to be the only available content. */
+	visibility: hidden;
 }
 
 .wp-full-overlay {
@@ -1613,8 +1615,10 @@ body.full-overlay-active {
 	height: 100%;
 }
 
-.customize-active #customize-container {
-	display: block;
+/* Make the Customizer and Theme installer overlays the only available content. */
+#customize-container,
+.theme-install-overlay {
+	visibility: visible;
 }
 
 .customize-loading #customize-container iframe {