FWIW (and I’m not sure where this comment should be added), I’ve seen numerous WP themes which do not include the proper body class on the Events Calendar pages. My theme, for example, extends the WP body_class and replaces it in my header.php, so I simply hooked into that extension and added the proper body classes in conditional statements as found in a typical ECP install.
I bring this up because the CSS element attributes for tooltips are specific to a particular body class. In this case, on the Month view, your body class should include “events-gridview,” otherwise the appropriate styles do not get applied. Furthermore, the ECP CSS is great and advanced, and the overrides work as prescribed, but I think a lot of folks have troubles understanding specificity in style sheets.