Index: src/wp-admin/css/colors/_admin.scss
===================================================================
--- src/wp-admin/css/colors/_admin.scss	(revision 29186)
+++ src/wp-admin/css/colors/_admin.scss	(working copy)
@@ -459,6 +459,8 @@
 	color: $menu-highlight-text;
 }
 
+.plugins-php .tb-close-icon:hover,
+.plugin-install-php .tb-close-icon:hover,
 .theme-install-overlay .close-full-overlay:hover,
 .theme-install-overlay .close-full-overlay:focus,
 .theme-install-overlay .previous-theme:hover,