Steps to reproduce the issue

Navigate to a third level menu item in the administrator interface (e.g. Content -> Articles -> Add New Article).

Expected result

Flyout menu link should appear with a solid background so that the underlying part of the interface does not show through it.

Actual result

Background, before hovering, is transparent and text is, depending on the location, unreadable or difficult to read.

System information (as much as possible)

Joomla: 3.6.2
Browser: tested on Firefox (Mac/Win), Chrome (Mac), Safari
PHP Version 5.5.14
Web Server Apache

Additional comments

Adding a background color to the isis template.css file is a workaround. It might also be missing a background image.

File: administrator/templates/isis/css/template.css
Either of these will do it but aren't exactly the fix:
.dropdown-menu > li > a {
background-color: #fefefe;
}
.dropdown-menu li {
background-color: #fefefe;
}