How to change the font of drop-down menu items?

In order to specify a different font size for the sub menu item, you could use below CSS snippet.

.sub-menu li a {
font-size: 10px !important;
}

This is how we could add the above sitewide CSS to your site.

Most of the layouts in the Flexile header pack also have a code module with some CSS snippets and you could add the above snippet there as well.