Source: http://www.elegantthemes.com/blog/community/divi-nation-short-3-super-simple-divi-header-modifications

This effect will work on the three classic Divi menu styles: default, centered, and centered inline. Add this CSS snippet to the custom CSS field in the Divi settings:

#top-menu li > a:after {
transition: all 0.4s ease-in-out;
}
#top-menu li:hover > a:after {
transform: rotate(360deg);
}Code language: CSS (css)