Themes: Allow use of sidebar submenus when theme details open.

Increase the z-index of sidebar submenus in the theme browser screens so that submenus are not placed behind the theme details overlay. Also set a min-height to ensure the overlay covers the entire screen if content is short.

Props codexdemon, amolebonde, yahil, girishpanchal, chintanmachhi207, mp518, janak007, worldweb, ralessio, sabernhardt, boogah, mikinc860, adamsilverstein, subrataemfluence, huzaifaalmesbah, lakshmananphp, amin7, imranhasanraaz, mdibrahimk48. 
Fixes #41155.

git-svn-id: https://develop.svn.wordpress.org/trunk@59821 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/themes.css

@@ -11,6 +11,10 @@
 	overflow-y: scroll;
 }
 
+.themes-php #adminmenuwrap {
+	z-index: 10001; /* above Theme Overlay */
+}
+
 body.js .theme-browser.search-loading {
 	display: none;
 }
@@ -372,6 +376,7 @@ body.js .theme-browser.search-loading {
 	background: #f0f0f1;
 	background: rgba(240, 240, 241, 0.9);
 	z-index: 10000; /* Over WP Pointers. */
+	min-height: calc(100vh - var(--wp-admin--admin-bar--height, 32px));
 }
 
 .theme-overlay .theme-header {