It looks like another plugin’s styling for tooltips is overriding Microthemer’s tooltip styling. They should have a dark blue background and no border. If you don’t mind sending me access detail to your site via our secure contact form I will identify the CSS conflict and update Microthemer’s code in the next update to make sure it doesn’t get overridden.
On a related note, you may also like to enable inline text labels for the individual CSS properties via View > Property text labels. Or using the keyboard shortcut: Ctrl + Alt + L.